3 "$\rm O-C$'' calculation

The "$\rm O-C$'' calculation for the collected times of maximum light of VZ Cnc was performed with the computer program OMC (Fu et al. 1998). First, a linear fit is made by the formula of $C_{\rm l}=T_1+P_{1}E$. The results of fitting are: T₁= HJD 2431550.7187, P₁=0.178363665 days, and $\sigma_1=0.0062$ days. Then, a quadratic curve is used to fit the times as $C_{\rm Q}=T_2+P_2E+0.5\beta E^2$. The fitting parameters are: T₂= HJD 2431550.7199, P₂=0.178363585 days, $\beta=7.9\ 10^{-13}$ days/cycle, and $\sigma_2=0.0062$ days. The (O-C)$\rm _l$ diagram and the fit curve using the quadratic function are shown in Fig. 1.

  

Figure 1: The $\rm(O-C)$ diagram and the fit curve by a parabolic function. Large scatter is found between the observation points and the fit curve

From Fig. 1, it may be found that the "$\rm O-C$'' calculation does not provide reasonable explanation on the period variation of VZ Cnc with the existence of large scatter for the fitting, and there may be more frequencies in the pulsation of this star since large scatter is present even in one observation season. In this case, the high-quality observation data sets listed in Table 2 are used to do multiple frequency analysis to study the pulsation of VZ Cnc.