COMMISSION 27 OF THE I. A. U.
             INFORMATION BULLETIN ON VARIABLE STARS
                          Number 3635

                                               Konkoly Observatory
                                               Budapest
                                               9 July 1991

                                               HU ISSN 0374 - 0676


              An improved, unambiguous period of the
                         Ap star 108 Aqr


The star 108 Aqr = HR 9031= HD 223640, classified B9pSiSrCr by Cowley et
al. 1969, has been observed in the Geneva photometric system between 1986 and
1988 at ESO La Silla with the 70 cm Swiss telescope.

The period (3.73 +- 0.03 days) published by Morrison & Wolff 1971, was not
satisfactory because it was based on uvby measurements taken only once a night,
so that the alias frequency 1-nu (i.e. P =1.37 days) was not excluded at all.
Our 64 Geneva observations allow to exclude clearly the alias period (P =1.37
days), and our V data combined with the 36 y data of Morrison & Wolff 1971,
yield the period

                P = 3.735236d +- 0.000024

while the maximum light in the V band occurs on HJD = 2444655.047. The
resulting lightcurve is shown in Fig. 1, together with the fitted curve (a
Fourier series truncated to the first harmonic). The r.m.s. residual scatter
of all 100 V and y data together around the fitted curve is only 0.0042
magnitudes.


                              [FIGURE 1]

 Figure 1: Lightcurve of 108 Aqr in the Geneva V band (crosses) and in 
           Stromgren's y band (open dots). See the ephemeris in the text.



P. North                                 M. Burnet
Institut d'astronomie                    Observatoire de Geneve
de l'Universite de Lausanne              ch. des Maillettes 51
CH-1290 Chavannes-des-Bois               CH-1290 Sauverny
Switzerland                              Switzerland

References

Cowley A., Cowley C., Jaschek M., Jaschek C., 1969, AJ. 74, 375 [BIBCODE 1969AJ.....74..375C ]
Morrison N.D., Wolff S.C., 1971, PASP 83, 474 [BIBCODE 1971PASP...83..474M ]