(S (S (BASENP (NNP Mkn) (NNP 421)) (VP (VBD was) (VP (VP (VP (VBN observed) (BASENP (NN three) (NNS times))) (PP (IN with) (NP (BASENP (NNP Chandra) (POS 's)) (NP (BASENP (NNP Low-Energy) (NNP Transmission) (NNP Grating)) (PRN (LRB -LRB-) (BASENP (NNP LETG)) (RRB -RRB-)))))) (COMMA ,) (ADVP (ADVP (ADVP (RB twice) (PP (IN in) (NP (BASENP (NN conjunction)) (PP (IN with) (BASENP (DT the) (NNP High) (NNP Resolution) (NNP Camera)))))) (PRN (LRB -LRB-) (NP (BASENP (NNP May) (NN 2000)) (CC and) (BASENP (NNP July) (NN 2003))) (RRB -RRB-))) (CC and) (ADVP (ADVP (RB once) (PP (IN with) (BASENP (DT the) (NNP Advanced) (NNP CCD) (NNP Imaging) (NNP Spectrometer)))) (PRN (LRB -LRB-) (BASENP (NNP October) (NN 2002)) (RRB -RRB-))))))) (PERIOD .))