thin disk (Shakura & Sunyaev 1973). The persistence of the optical continuum and emission lines suggests that this transition radius is somewhere between the regions that are most responsible for the soft X-rays and the H-ionizing continuum. We comment on this further in section 4.3 below.

The Virial Mass and Implications for NLS1s
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As noted above, reverberation-based size estimates for the broad emission lines and resulting virial mass estimates provide a potential means of distinguishing among the various NLS1 models. In Fig. 6, we show the relationship between the BLR radius as measured from the  lag as a function of the optical continuum luminosity for all AGNs with Balmer line lags known to reasonable accuracy. All data used here are taken from the compilation of Kaspi et al. (2000), though their parameters for NGC 4051 are superceded by the values reported here. This compilation contains six additional AGNs that could in principle be classified as NLS1s as they meet the criterion $\vFWHM \ltsim 2000$. These sources, which we shall refer to below as “narrow-line objects”, are the Seyfert galaxies Mrk 335 and Mrk 110 (from Wandel, Peterson, & Malkan 1999) and the QSOs PG 0026$+$120, PG 1211$+$143, PG 1351$+$640, 