Our plan was to collect enough data on this star to allow us to build up a complete light curve during the 7 nights available at the VSA 97. This was in order to provide some more data useful to decide if this star belongs to the very exclusive class of SX Phe stars or not. Unfortunately, due to the bad sky conditions, we were allowed to gather magnitude measurements using only 8 hours of telescope time, not enough to get a complete light curve. Our data shows that the general trend in the lightcurve is similar for both filters: B and V. Some comments about it:
1) the accuracy of our data is in the range 0.03 magnitudes for the V filter and 0.05 magnitudes for the B filter. There are some scattered points in our lightcurve because of the bad sky conditions we found during some observing sessions (clouds passing over our star field, etc.). Despite this, the general lightcurve can be traced out.
2) it seems that the peak to valley variation in the B filter is larger compared to the V filter variation (1.1 magnitude drop vs 0.8 magnitude drop); this can be explained by a subtle variation in the color index of the star during its cycle. If this is true, it could be a key to rule out XX Cyg from the SX Phe stars, because these stars are known to keeptheir color index almost constant along their whole light curve.
3) another interesting thing that could be clearly noticed from our lightcurve is that the instant of the maximum as we found it in the literature does not coincide with the one we found with our measurements. This could mean that: i) the ephemerids we found are too old to be used today, or ii) the star has changed its pulsation cycle somehow in the past. However, we need some more data to establish new ephemerids that could explain this difference.
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