method of Gaskell & Sparke (1986) and Gaskell & Peterson (1987) and the discrete correlation function (DCF) method of Edelson & Krolik (1988), where in both cases, we employ the specific implementation described by White & Peterson (1994).

The results of the cross-correlation analysis are summarized in Table 4 and the cross-correlation functions are shown in Fig. 4. For both  and 686, Table 4 gives the ICCF centroid , and the location $\tau_{\rm peak}$ of the maximum value of the correlation coefficient $r_{\rm max}$. The centroid is computed using all points near $\tau_{\rm peak}$ with values greater than 0.8$r_{\rm max}$. The uncertainties quoted for  and $\tau_{\rm peak}$ are based on the model-independent Monte-Carlo method described by Peterson et al. (1998).

By combining this lag with the Doppler width of the emission line, we can estimate a virial mass, as in eq. (4); for consistency with Wandel et al. (1999) and Kaspi et al. (2000), we use $f=3/\sqrt{2}$ in eq. (4). Since the broad emission-line features are comprised of a number of different components (or contaminants), it is desirable to measure the Doppler width of only the [*variable*]{} part of the emission line. In order to isolate the variable part of 