5. Other variables
Of remaining four variables, the most interesting is Oo 922 (BD +56 o515) which appears to be a periodic one. The star was classified as B 0.5Vn by Schild (1965). Periodogram analysis of the 1994-1996 data of Oo 922 revealed a single peak at frequency 1.380 d-1. The period, derived from the most numerous V-filter data, was found to be 0.724536 0.000012 d. Within the errors, the semi-amplitudes are almost the same in all three bands of the system, amounting to 6.8 0.6, 5.8 0.3, and 6.1 0.2 mmag for U, B, and , respectively. In the 1997 data, however, the 0.725-day periodicity can barely be found; the changes seem to be rather aperiodic. From the characteristics of the star's variations, Oo 922 could be a Eri-type variable. Fortunately, in late 1989 and early 1990 the star was searched for the presence of the H emission by Goderya & Schmidt (1994). Their observations were carried out on two epochs separated by about three months. The first observation did not indicate the presence of emission, but the second one did. The H observations of h Persei were also carried out in Bial ków in the beginning of 1997, and no H emission was found in Oo 922 (Pigulski et al., in preparation). It seems that Oo 922 is a Be star with emission which sometimes weakens to a level undetectable by photometric methods. It should be also remembered that Eri-type photometric variations are sometimes observed in Bn stars without emission (see Balona 1990).
The brightest star in the field, the cluster supergiant Oo 1057 (HD 14134, BD +56o522, V 520 Per), was classified as a B3 Ia star by Johnson & Morgan (1955) and as a B3 Iab star by Slettebak (1968). The star was suspected to be variable by Rufener & Bartholdi (1982) on the basis of their Geneva photometry. Its variability was later confirmed by Waelkens et al. (1990). Oo 1057 was also observed by the HIPPARCOS satellite. Although light changes are clearly visible both in the HIPPARCOS and our data, we failed to find any well-defined periodicity. The variations seem to be irregular with a range of about 0.15 mag in HIPPARCOS , and 0.06 mag in our data.
In the periodogram of the 1994-1996 data of Oo 986, the strongest peak appears at a frequency of 1.87622(5) d-1. After prewhitening with this frequency, the next strongest peak occurs at = 0.62529(7) d-1, which is commensurable with , i.e., . The cause of the variability is unknown.
Percy (1972) flagged the variability of the B2 Vn star Oo 1080 as `uncertain' and gave an upper limit of 50 mmag for its photometric -filter variations. Our observations confirm that the star is indeed variable. Analysis of all our -filter data yielded the periodicity of 1.49752 d. However, phasing with twice period (2.99504 d) results in a slightly smaller scatter. If the longer period is real, the most reliable explanation is that the star is ellipsoidal. Nevertheless, the possibility that Oo 1080 is an SPB, or even Eri variable similar to Oo 922, cannot be rejected. Assuming a 1.5-day period, the semi-amplitudes we derived were 9.9 1.6, 6.0 0.5, and 7.7 0.3 for U, B, and V, respectively.
Three other early-B stars suspected to be variable by Percy (1972), namely Oo 963 (NSV 776), Oo 978 (NSV 777), and Oo 1004 (NSV 778), appeared to be constant. The detection thresholds for the full amplitude of periodic variations in for these three stars were 5, 6, and 6 mmag, respectively. Another of Percy's suspects, Oo 1161 (NSV 781), was slightly outside the observed field.
© European Southern Observatory (ESO) 1999
Online publication: April 19, 1999