4 HR 7786 and HR 7721

Winzer (1974) discovered HR 7786 (= HD 193722 = V1584 Cyg) to be a large amplitude variable (0.11 mag in U). He derived a period of 1.13254 days and used 30 Cyg as his comparison star. Many of our FCAPT observations were apparently no good if we demanded that our ch-c values remained constant. Using 9 FCAPT observations in 1990-91, 19 in 1992-93, 18 in 1993-94, and 44 in 1994-95 a periodogram analysis showed that the most probable period was 8.5259 days. Comparison of our y data with Winzer's V values showed that there was a small phase shift. Thus the period was found to be $8.5297\ \pm\ 0.0002$ days. As light maxima are sharper than the broad minima, we selected the zero epoch to fall within Winzer's observations, $2441618.86\ \pm\ 0.02$. After the Hipparcos data (ESA 1997) become available (see Table 5), it was apparent that the comparison star HR 7721, a known spectroscopic binary, (Hoffleit 1982) was variable.

To improve the quality of our light curves and to attempt to find the nature of HR 7721's variability, we plotted our v-c data according to our period. Then we deleted values which were clearly in error. This left 21 FCAPT values in 1990-91, 39 in 1992-93, 39 in 1993-94, and 107 in 1994-95, numbers which are substantially greater than those used to find the period.

Figure 2 shows that the uvby light curves are approximately in phase. The V observations of Winzer (closed circles) as normalized to the y observation zero point overlay the y FCAPT values (pluses). There are suggestions of structure in the light curve. The amplitude in u is 0.13 mag, and in v, b, and y 0.065 mag. Thus the energy distribution just above the Balmer jump remains relatively constant.

  

Figure 2: Photometry of HR 7786 plotted according to this paper's ephemeris. The FCAPT observations are indicated as plus signs and Winzer's V magnitudes transformed to y as closed circles

HR 7721 is a spectroscopic binary with a period of 7.18584 days (Hube & Lowe 1980). When we used a Scargle periodogram on the y(ch-c) data we found a period of either 1.10267 or 10.0733 days. Plotting the data against phases based on these two periods or the binary period showed mostly scatter. For 10.0733 days, we saw the suggestion of a modulation with about 0.1 magnitude of scatter. When we look only at one season of photometry, for some magnitudes we see suggestions of a semi-periodic variability, but not for others using these three different period. Figure 3 shows the b photometry for the 1994-95 observing season. As HR 7721 is a B7 V star, at least one of its components might be a slowly pulsating B star. Clearly this is a star which requires more study.

  

Figure 3: The b photometry of HR 7721 plotted as a function of Heliocentric Julian Date for the 1994-95 observing season