All Figures

$\begin{figure} \par\includegraphics[scale=0.47]{8596fig1.eps} \end{figure}$ Figure 1: The O-C diagram of HT Cas with respect to the linear ephemeris of Table 3. The individual timings from the literature and eclipse timings measured from our light curves are shown as open squares, while the average timings of Table 2 are denoted by solid circles. The dotted line in the upper panel depicts the best-fit quadratic ephemeris while the solid line in the middle panel shows the best-fit linear plus sinusoidal ephemeris of Table 3. The lower panel displays the residuals with respect to the linear plus sinusoidal ephemeris of the middle panel.
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In the text

$\begin{figure} \par\includegraphics[scale=0.47]{8596fig2.eps} \end{figure}$ Figure 2: Influence of data sampling ( upper panel), time coverage ( middle panel) and accuracy of the eclipse timings ( lower panel) in the detection of orbital period modulations. The dotted line represents (O-C) values constructed from a period modulation of 20 yr and amplitude 50 s (20 s in the lower panel). Gaussian noise of amplitudes 5 s ( upper and middle panels) and 20 s ( lower panel) were added to the (annually sampled) timings to simulate the uncertainties of a real data set. The solid line depicts the best-fit ephemeris obtained for the synthetic data used (solid circles) to illustrate each case.
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In the text

$\begin{figure} \par\includegraphics[scale=0.36,angle=270]{8596fig3.eps} \end{figure}$ Figure 3: Diagram of the fractional period change $\Delta P/P$ versus the angular velocity $\Omega$ of the active component star for CVs (open circles) and other close binaries from the literature (crosses). Median values of $\Delta P/P$ for the CVs above and below the gap and for the other binaries (in bins of 0.25 dex in $\log\Omega$ ) are shown as filled squares with error bars. The period gap is indicated by vertical dashed lines. The best-fit linear relation and its 3- $\sigma$ confidence level are shown as solid and dotted lines, respectively.
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In the text