COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 1728

                                    Konkoly Observatory
                                    Budapest
                                    1980 January 14


       HR 7308, A SHORT-PERIOD CEPHEID WITH VARIABLE AMPLITUDE


HR 7308, (=BD+27d3314) is a F6Ib-II star of 6th magnitude. Semi-regular
photometric variations were detected by Breger (1969) and Percy et al. (1979)
find that this star is a new small-amplitude cepheid. The radial velocity RV
of HR 7308 was measured 132 times with the spectrophotometer CORAVEL (Baranne
et al., 1979) between May 1977 and November 1979, and 10 photometric
measurements in the Geneva system were obtained between October 1978 and May
1979. Figure 1 presents the RV data in function of the Julian Date. The
measurement uncertainty, resulting from the photon noise, the scintillation and
instrumental causes, is typically 0.43 km/sec in the case of HR 7308.

                             [FIGURE 1]

Figure 1 shows that the RV measurements are characterized by :

1) A short-period variability (RV=-9km/sec at JD 2443741.6 and -22 km/sec at JD
2443742.4)

2)An amplitude of variation which is variable with time. During the period of
survey, the peak-to-peak amplitude has decreased from 18 km/sec at JD 2443710
to 4 km/sec at JD 2444000.

The search for the period of the short-term variability by a method of Fourier
analysis (Burki and Rufener, 1978) reveals that a single period

P_0=1.49107 d.

characterizes all RV data. Figures 2 and 3 show the RV curves calculated with
P_0 and the phase origin JD 2440000, for 2 different epochs of survey.

                             [FIGURE 2]

In Figure 2, the symbols plus refer to the data in the interval
2443706<JD<2443719 and the crosses to the data in the interval
2443726<JD<2443766. Figure 4 shows the light curve in magnitude V for the same
period P_0 and the same phase origin. The squares refer to the data in the
interval 2443788<JD<2443802 and the open circles to the data in the interval
2443971<JD<2444001. We see that the light curve is the mirror image of the RV
curve.

                             [FIGURE 3]
                             [FIGURE 4]

Thus, HR 7308 is a new short-period cepheid exhibiting a strongly variable
amplitude. An analysis of the properties of this bright star reveals that it
probably belongs to the population I. If this is really the case, HR 7308
would be the shortest classical cepheid known in our Galaxy. We shall continue
to follow this star, both in radial velocity and photometry, in order to
describe the future variations of the amplitude and, thus to determine its
evolutionary status.


G. BURKI, M. MAYOR
Geneva Observatory
CH-1290 Sauverny
Switzerland

References:

Baranne, A., Mayor, M., Poncet, J.L.: 1979, Vistas in Astronomy, in press [BIBCODE 1979VA.....23..279B ]
Breger, M. : 1969, Astrophys. J. Suppl. 19, 79 [BIBCODE 1969ApJS...19...79B ]
Burki, G., Rufener, F.: 1978, Astron. Astrophys. 70, 105 [BIBCODE 1978A&A....70..105B ]
Percy, J.R., Baskerville, I., Trevorrow, D.W.: 1979, Publ.Astron. Soc. Pacific
   91, 368 [BIBCODE 1979PASP...91..368P ]