6 Photometric variability of Be stars

As we mentioned above, 14 Be stars from Table 1 are situated in the V field, the target of the variability survey. Out of these 14 stars, 10 were found to be variable (Pigulski et al. 2001) and only four (Sanduleak 11, 17, 28, and the newly discovered Be star, W128) show no light variation above the detection limit. Nine variable Be stars show aperiodic or quasiperiodic variatons with ranges up to 0.4 mag in $I_{\rm C}$. Only Sanduleak 10 is a periodic variable of $\lambda$ Eridani type with a period of 0.67298 d (Pigulski et al. 2001). Two drops of brightness seen in the light curve of Sanduleak 29 on HJD2451200 and 2451439 could be eclipses. A large percentage of variables among Be stars is not unusual, because virtually all these stars exhibit some degree of photometric variability on different time-scales. The light curves of all variable Be stars situated in the V field are shown in Fig. 6.

Although observed on only two or three nights, some of the Be stars from outside the V field can also be classified as variables. These are: Sanduleak 1 ( $\Delta I_{\rm C} \approx 0.39$ mag), 2 ( $\Delta I_{\rm C} \approx$ 0.09 mag), 3 ( $\Delta I_{\rm C} \approx$ 0.10 mag), 4 ( $\Delta I_{\rm C} \approx$ 0.10 mag), and 23 ( $\Delta I_{\rm C} \approx$ 0.05 mag). The remaining Be stars, that is, Sanduleak 11, 21, 22, 25, 26, W128, 151, and 240, show no clear evidence of variability exceeding the photometric errors of our photometry.

Figure 7: CM diagram for NGC663. Crosses denote stars identified as non-members (see Sect. 7). Diamonds are used to indicate the emission-line objects. The dashed line shows the limit of the H$\alpha$ photometry.

In order to plot an average position of the variable Be star in the CM diagram (Fig. 7), one should use observations made simultaneously in all bands. Such observations were chosen for averaging, but the epochs of the averaged magnitudes are not the same for all Be variables (different fields were observed on different nights). These average epochs of the $BV(RI)_{\rm C}$ photometry are the following: HJD2450515.3 for Sanduleak 9, 14, and 16; HJD2450518.4 for Sanduleak 13; HJD2450813.3 for Sanduleak 1, 2, and 4; HJD2450845.3 for Sanduleak 23; HJD2450846.3 for Sanduleak 5, 6, 8, 12, and 29; and HJD2451058.6 for Sanduleak 3. Observations of non-variable Be stars and Sanduleak 10 ($\lambda$ Eridani-type variable) were averaged using all available photometry.