user315: A question of rulebook pedantry:

One can claim a draw when the same position occurs the third time, the same player having the move.
Two positions are not the same if the earlier allows a capture en passant and the later does not, though men of the same kind stand on the same squares in both positions.

So much I understand.  However, what if the pawn that could otherwise have captured en passant happens to be pinned against its own king?  In this case, one might say that the pawn had the right to capture en passant but could not exercise it.  Alternately, one might insist that a right that cannot be exercised is in fact no right.
I suppose that it comes to how one defines the word right.
The reason this is a question of rulebook pedantry is because it might, conceivably, legally affect whether one could claim a draw if the position recurred.  For the purpose of the draw rule, are two positions indeed the same if, in the earlier position, a pawn had the right to capture en passant but could not exercise it because of a pin?
To be clear:  My question does not regard how fantastically unlikely such a situation would be ever to arise in actual play, nor if it has ever occurred.  My question regards only what the rule would be if it did.
Your answer merits extra appreciation if you can support it with any reasonably authoritative reference!
THIRD UPDATE
I know no reason that anyone should be following these updates any longer -- and I rather suspect that no one is reading (if you are not reading, you are probably wise) -- but, for the permanent record and in response to a reader's reasonable request, I have deleted parts of the second and first updates.  If the deletions still don't suit, further edits by interested others naturally remain welcome.
SECOND UPDATE
@CraigYoung characterizes the first update [as originally written] as a "diatribe."  I think that he has a point.  A Stackexchange question is not the right forum for the asker (me) to extemporize on the philosophy of rulebook writing.  As originally written, above, the question was indeed just a question -- and was not originally meant as anything other than a request for information.  What the answers seem to me to have revealed however is that no authoritative answer is possible at the present time.  The answers occasioned a discussion, and the discussion seemed worth preserving, whence the first update.
I called the original question "pedantic" to poke a bit of deserved fun at myself, of course.  My inner David Hilbert, as it were, wants the rules of chess to be logically complete; but the real, original David Hilbert was a wise man, a deep mathematician but also a shrewd judge of his audience.  He could approach such questions with a sense of humor and a light touch, understanding that the matter of logical completeness would not fascinate everyone as much as it did him.  Dr. Hilbert saved his earnestness for audiences prepared to appreciate it.
If you will let me don the cloak of earnestness for a moment:  Each chess player can judge for himself how much or how little chess loses by failing logically to complete its rules, but surely chess gains nothing by this failure.  Chess has a body, the FIDE, whose reasonable, permanent decisions in such matters would command broad respect, except that the FIDE seems mired in endless scandal and has never clearly rendered the decisions in question.  So, if you're a Hilbert type like me, you're stuck, aren't you?
There is no rule.  There could be.  There should be.  But there isn't.  I admit that I don't like that.
FIRST UPDATE
@JamesTomasino's fine answer below is recommended to all interested readers.  His logic looks pretty good, his angle is attractive, his judgment seems sound, and he may be right.  I do not disagree with him, but I am not yet convinced.
The reason is this.  There exists a school of rulebook construction with which I admittedly do not hold (and to which I do not suggest that @JamesTomasino belongs).  In that school, a rule is to be enforced even if the rule's writer never considered the point to which the rule is applied.  More precisely, the rule's language is to be enforced.
Actually, this might be a tolerable way to construe a rulebook, except that we have ample experience to suggest that, in cases such as this, two, different, reasonable people will infer two, different rules from the same, flawed language.  Each reasonably believes that he stands upon "the rock of the law."  The rules of chess however are to be a logical construct, not a linguistic one.  If we are arguing over what the words mean, and if no consensus exists, then it is the words that are at fault.  Such words shirk their duty.
Anderssen v. Kieseritzky may be poetic.  The rulebook by which they play however is supposed to be precise.  If it is not, then this is a real lack in the rulebook.
Here again is the useful reference @JamesTomasino has kindly provided to the FIDE handbook, section 9.2.  After reading it thrice, I can only conclude that the rule is [hard for me to understand.]  What are "the possible moves of all the pieces of both players"?  Taken literally, "all the pieces" (what ever happened to the traditional phrase "all the men"? but let that pass) of one player have no moves at all, since it is his opponent's turn.  Now, of course, obviously, that is not what the rule means, but what it does mean is far from clear.
To muddy the waters further, see the interesting discussion between USCF TDs @Andrew has discovered and brought to our attention.
It seems to me that the answer we are reaching here is that there exists no generally agreed-upon interpretation of the rules in these corner cases.  That's too bad.  I know what I think the rule should be, but what I think is irrelevant to this discussion, because I am no Staunton or Steinitz and no one cares (or should care) what I think.  My chief concern is not whether this rule or that rule might be the better, but only that there in fact be a rule.
Actually, it seems to me that there is a deeper problem.  The FIDE Handbook differs from the equivalent publication of the American Contract Bridge League in that the latter draws a clear distinction between the mechanics of the game and guidelines of player conduct.  In the FIDE Handbook, the two are sort of all mixed up together.

user17: A pawn that is captured "en passant," starts on the second rank and ends on the fourth rank, having gotten there via the third rank.
The "en passant" rule allows the opponent to stop the pawn on the third rank, where he can be captured. (In war, if one army starts to march, it might provoke a "fire" from the opposing army.)
In a three move repetition rule, the pawn on the fourth rank started there, because it had not been captured en passant. If you have three repetitions that lead to a draw, it's because all pieces started on the same square.
The three move rule specifically applies when no pawn had been moved and no pawn or piece captured. An "en passant" situation is a violation of both these conditions.

user10591: You never know when such rules will become relevant. In the past you could seal an illegal move (back when adjournments occurred), but that gave the player a chance to seal such an illegal move and then spend his adjournment time thinking of options. There was a puzzle I saw once which emphasized castling is definitely on a rank by showing how to castle up the board Ke1-e3 and Re8-e1 to give checkmate (notation: O-O-O-O-O-O-O-O).
More real was the problem Viktor Korchnoi had in one of his world championship matches with Anatoly Karpov. He wanted to castle queen-side as Black, but Karpov had a bishop on the h2-b8 diagonal, controlling b8. Could he castle? He couldn't recall the rule and had to ask the arbiter.

user8201: Thanks for the question, and for the very competent answers. It's not pedantry: it's a genuine corner-case for chess-engine programmers and retrograde analysis enthusiasts. A couple of years ago I asked international arbiter Stewart Reuben (FIDE rulebook committee chairman) who confirmed that the rule works as James Tomasino surmised: i.e. if, because of some pin or check, an en passant capture was never possible, then that position is considered to be the same as later copies.

user25142: Following the thread you linked and doing some digging from there, I found a PDF of a 2011 Arbiter's Notebook column which addresses this issue.

Although the player of the
white pieces will lose the right to castle in both diagrams on the next move,
the rule states that in the actual position the player has not yet lost the right to
castle. This means that if same positions appear again on the board, these
positions will be considered as different.

His first example position appears to have been butchered at some point in its hosting history, but the second one seems to be intact and makes his point clear; the "right" to castle contributes to the position's identity even though castling is an illegal move. I see no reason to suppose this does not extend to en passant, contrary to the other answers here.
Whether this "illegal right to castle" also covers positions where there is a piece between the rook and the King is unclear to me. I suspect Gijissen would say it does not, but I find it hard to accept that illegal movement through check is fundamentally different from illegal movement through pieces.

user11245: The question hinges on whether an "illegal" move is a "possible" move.  Strictly speaking, the move is not possible, any more than Bc1-c3 is possible.  Both are "illegal" moves.
In the first position, there is no en passant capture that is possible, hence

"... the possible moves of all the pieces of both players are the same."

Until FIDE draws a distinction between these two terms, then, the first position is identical to the later positions, and the draw claim would be valid.
I should add, however, that I am not a FIDE-certified Arbiter. An Arbiter would be the definitive source for an answer to your question.

user29: The basis of your question comes down to whether a piece is truly in position to execute an en passant if it is otherwised pinned. The answer can be found in the FIDE rules stated just before the explicit mention of the en passant:

"Positions as in (a) and (b) are considered the same, if the same player has the move, pieces of the same kind and colour occupy the same squares, and the possible moves of all the pieces of both players are the same."

The key point is "the possible moves of all the pieces of both players are the same". The en passant passage is mentioned following this to give an explicit example one might overlook.
If in the first occurrence of the position the en passant was not an available move due to another limiting factor, then the list of all available moves would be identical when it appears again.
Thus, draw!
You can find the details in the FIDE handbook, section 9.2.

315: <p>A question of rulebook pedantry:</p>

<ol>
<li>One can claim a draw when the same position occurs the third time, the same player having the move.</li>
<li>Two positions are not the same if the earlier allows a capture <em>en passant</em> and the later does not, though men of the same kind stand on the same squares in both positions.</li>
</ol>

<p>So much I understand.  However, what if the pawn that could otherwise have captured <em>en passant</em> happens to be pinned against its own king?  In this case, one might say that the pawn had the right to capture <em>en passant</em> but could not exercise it.  Alternately, one might insist that a right that cannot be exercised is in fact no right.</p>

<p>I suppose that it comes to how one defines the word <em>right.</em></p>

<p>The reason this is a question of rulebook pedantry is because it might, conceivably, legally affect whether one could claim a draw if the position recurred.  For the purpose of the draw rule, are two positions indeed the same if, in the earlier position, a pawn had the right to capture <em>en passant but could not exercise it</em> because of a pin?</p>

<p>To be clear:  My question does not regard how fantastically unlikely such a situation would be ever to arise in actual play, nor if it has ever occurred.  My question regards only what the rule would be if it did.</p>

<p>Your answer merits extra appreciation if you can support it with any reasonably authoritative reference!</p>

<p><strong>THIRD UPDATE</strong></p>

<p>I know no reason that anyone should be following these updates any longer -- and I rather suspect that no one is reading (if <em>you</em> are not reading, you are probably wise) -- but, for the permanent record and in response to a reader's reasonable request, I have deleted parts of the second and first updates.  If the deletions still don't suit, further edits by interested others naturally remain welcome.</p>

<p><strong>SECOND UPDATE</strong></p>

<p>@CraigYoung characterizes the first update [as originally written] as a "diatribe."  I think that he has a point.  A Stackexchange question is not the right forum for the asker (me) to extemporize on the philosophy of rulebook writing.  As originally written, above, the question was indeed just a question -- and was not originally meant as anything other than a request for information.  What the answers seem to me to have revealed however is that no authoritative answer is possible at the present time.  The answers occasioned a discussion, and the discussion seemed worth preserving, whence the first update.</p>

<p>I called the original question "pedantic" to poke a bit of deserved fun at myself, of course.  My inner David Hilbert, as it were, wants the rules of chess to be logically complete; but the real, original David Hilbert was a wise man, a deep mathematician but also a shrewd judge of his audience.  He could approach such questions with a sense of humor and a light touch, understanding that the matter of logical completeness would not fascinate everyone as much as it did him.  Dr. Hilbert saved his earnestness for audiences prepared to appreciate it.</p>

<p>If you will let me don the cloak of earnestness for a moment:  Each chess player can judge for himself how much or how little chess loses by failing logically to complete its rules, but surely chess <em>gains</em> nothing by this failure.  Chess has a body, the FIDE, whose reasonable, permanent decisions in such matters would command broad respect, except that the FIDE seems mired in endless scandal and has never clearly rendered the decisions in question.  So, if you're a Hilbert type like me, you're stuck, aren't you?</p>

<p>There is no rule.  There could be.  There should be.  But there isn't.  I admit that I don't like that.</p>

<p><strong>FIRST UPDATE</strong></p>

<p>@JamesTomasino's fine answer below is recommended to all interested readers.  His logic looks pretty good, his angle is attractive, his judgment seems sound, and he may be right.  I do not disagree with him, but I am not yet convinced.</p>

<p>The reason is this.  There exists a school of rulebook construction with which I admittedly do not hold (and to which I do not suggest that @JamesTomasino belongs).  In that school, a rule is to be enforced even if the rule's writer never considered the point to which the rule is applied.  More precisely, the rule's <em>language</em> is to be enforced.</p>

<p>Actually, this might be a tolerable way to construe a rulebook, except that we have ample experience to suggest that, in cases such as this, two, different, reasonable people will infer two, different rules from the same, flawed language.  Each reasonably believes that he stands upon "the rock of the law."  The rules of chess however are to be a logical construct, not a linguistic one.  If we are arguing over what the words mean, and if no consensus exists, then it is the words that are at fault.  Such words shirk their duty.</p>

<p>Anderssen v. Kieseritzky may be poetic.  The rulebook by which they play however is supposed to be precise.  If it is not, then this is a real lack in the rulebook.</p>

<p>Here again is the useful reference @JamesTomasino has kindly provided to the <a href="http://www.fide.com/fide/handbook?id=124&amp;view=article" rel="noreferrer">FIDE handbook, section 9.2.</a>  After reading it thrice, I can only conclude that the rule is [hard for me to understand.]  What are "the possible moves of all the pieces of both players"?  Taken literally, "all the pieces" (what ever happened to the traditional phrase "all the men"? but let that pass) of one player have no moves at all, since it is his opponent's turn.  Now, of course, obviously, that is not what the rule means, but what it <em>does</em> mean is far from clear.</p>

<p>To muddy the waters further, see <a href="http://main.uschess.org/forums/viewtopic.php?f=2&amp;t=6113" rel="noreferrer">the interesting discussion</a> between USCF TDs @Andrew has discovered and brought to our attention.</p>

<p>It seems to me that the answer we are reaching here is that there exists no generally agreed-upon interpretation of the rules in these corner cases.  That's too bad.  I know what I think the rule should be, but what I think is irrelevant to this discussion, because I am no Staunton or Steinitz and no one cares (or should care) what I think.  My chief concern is not whether <em>this</em> rule or <em>that</em> rule might be the better, but only that there in fact be a rule.</p>

<p>Actually, it seems to me that there is a deeper problem.  The FIDE Handbook differs from the equivalent publication of the American Contract Bridge League in that the latter draws a clear distinction between the mechanics of the game and guidelines of player conduct.  In the FIDE Handbook, the two are sort of all mixed up together.</p>
 17: <p>A pawn that is captured &quot;en passant,&quot; starts on the second rank and ends on the fourth rank, having gotten there via the <em>third</em> rank.</p>
<p>The &quot;en passant&quot; rule allows the opponent to <em>stop</em> the pawn on the third rank, where he can be captured. (In war, if one army starts to march, it might provoke a &quot;fire&quot; from the opposing army.)</p>
<p>In a three move repetition rule, the pawn on the fourth rank <em>started</em> there, because it had <em>not</em> been captured en passant. If you have three repetitions that lead to a draw, it's because <em>all</em> pieces started on the same square.</p>
<p>The three move rule specifically applies when <em>no</em> pawn had been moved and no pawn or piece captured. An &quot;en passant&quot; situation is a violation of both these conditions.</p>
 10591: <p>You never know when such rules will become relevant. In the past you could seal an illegal move (back when adjournments occurred), but that gave the player a chance to seal such an illegal move and then spend his adjournment time thinking of options. There was a puzzle I saw once which emphasized castling is definitely on a rank by showing how to castle up the board Ke1-e3 and Re8-e1 to give checkmate (notation: O-O-O-O-O-O-O-O).</p>

<p>More real was the problem Viktor Korchnoi had in one of his world championship matches with Anatoly Karpov. He wanted to castle queen-side as Black, but Karpov had a bishop on the h2-b8 diagonal, controlling b8. Could he castle? He couldn't recall the rule and had to ask the arbiter.</p>
 8201: <p>Thanks for the question, and for the very competent answers. It's not pedantry: it's a genuine corner-case for chess-engine programmers and retrograde analysis enthusiasts. A couple of years ago I asked international arbiter Stewart Reuben (FIDE rulebook committee chairman) who confirmed that the rule works as James Tomasino surmised: i.e. if, because of some pin or check, an en passant capture was never possible, then that position is considered to be the same as later copies.</p>
 25142: <p>Following the thread you linked and doing some digging from there, I found a PDF of a <a href="http://arbitri.lombardiascacchi.com/Chesscafe/2011_01.pdf" rel="nofollow noreferrer">2011 Arbiter's Notebook column</a> which addresses this issue.</p>
<blockquote>
<p>Although the player of the
white pieces will lose the right to castle in both diagrams on the next move,
the rule states that in the actual position the player has not yet lost the right to
castle. This means that if same positions appear again on the board, these
positions will be considered as different.</p>
</blockquote>
<p>His first example position appears to have been butchered at some point in its hosting history, but the second one seems to be intact and makes his point clear; the &quot;right&quot; to castle contributes to the position's identity even though castling is an illegal move. I see no reason to suppose this does not extend to en passant, contrary to the other answers here.</p>
<p>Whether this &quot;illegal right to castle&quot; also covers positions where there is a piece between the rook and the King is unclear to me. I suspect Gijissen would say it does not, but I find it hard to accept that illegal movement through check is fundamentally different from illegal movement through pieces.</p>
 11245: <p>The question hinges on whether an "<em>illegal</em>" move is a "<em>possible</em>" move.  Strictly speaking, the move is <em>not possible</em>, any more than Bc1-c3 is possible.  Both are "<em>illegal</em>" moves.</p>

<p>In the first position, there is no en passant capture that is possible, hence</p>

<blockquote>
  <p>"... the possible moves of all the pieces of both players are the same."</p>
</blockquote>

<p>Until FIDE draws a distinction between these two terms, then, the first position is identical to the later positions, and the draw claim would be valid.</p>

<p>I should add, however, that I am not a FIDE-certified Arbiter. An Arbiter would be the definitive source for an answer to your question.</p>
 29: <p>The basis of your question comes down to whether a piece is truly in position to execute an en passant if it is otherwised pinned. The answer can be found in the FIDE rules stated just before the explicit mention of the en passant:</p>
<blockquote>
<p>&quot;Positions as in (a) and (b) are considered the same, if the same player has the move, pieces of the same kind and colour occupy the same squares, and the possible moves of all the pieces of both players are the same.&quot;</p>
</blockquote>
<p>The key point is &quot;the possible moves of all the pieces of both players are the same&quot;. The en passant passage is mentioned following this to give an explicit example one might overlook.</p>
<p>If in the first occurrence of the position the en passant was not an available move due to another limiting factor, then the list of all available moves would be identical when it appears again.</p>
<p>Thus, draw!</p>
<p>You can find the details in the <a href="http://www.fide.com/fide/handbook?id=124&amp;view=article" rel="nofollow noreferrer">FIDE handbook, section 9.2.</a></p>
