COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 2093

                                 Konkoly Observatory
                                 Budapest
                                 1982 March 1

                                 HU ISSN 0374-0676




            S ERIDANI - A DELTA SCUTI VARIABLE




    S Eridani (= 64 Eridani = HR 1611 = HD 32045) was noted by Millis
(1967) as a variable of unknown type. Leung (1970)included this star
in his list of known Delta Scuti variables but this classification has been
questioned by Valtier (1972) who remarked that there was no evidence
of periodicity in Millis' limited data. S Eridani is included in the
1974 supplement to the GCVS as a possible Delta Scuti type star with a
visual light range of 0.02 magnitudes and a spectral type F0 IV; no period
is given. Eggen (1979) includes this star in his list of Ultra Short
Period Cepheids while Breger (1979) omits it from his Delta Scuti list.
    We felt that fresh observations of S Eridani may help establish
what type of variable it is and assign a period to it. Using the 40cm
reflector at Siding Spring Observatory we observed S Eridani on the 30th
November 1981. Two nearby standard stars, HR 1661 (= HD 32996) an A0
type star, and HR 1665 (= HD 33093) an F8 star, were chosen. All three stars
were measured in the V band of the standard UBVRI system through a large
range of air mass so that the differential measurements of magnitude could
be corrected accurately for atmospheric extinction.

       Figure 1 illustrates the light curve of S Eridani for the
observing run and the constancy of the standard stars during this time.
    Crawford et al.(1970) made uvby 8 measurements of S Eridani
and derived the following indices:  b-y = 0.171; m_1 = 0.169; c_1 = 1.002
and Beta = 2.754. Trigonometric parallax has been measured for this star
and reddening can be considered negligible. Using the measured c_1 and b-y
indices, the absolute magnitude, M_v, was calculated to be 0.8. Measured parallax
(0.015") for S Eridani places the star at about 67 parsecs. Though errors
in trigonometric parallax measurement at this distance are large,it is
worth noting that from. the known parallax, M_v is calculated to be 0.7.

                            [FIGURE 1]

    From our data (see figure 1) S Eridani appears to have a longer
period than most small amplitude Delta Scuti stars. Estimation of a period from
a single night's data can be hazardous,but using Fourier techniques we suggest
that this star has a fundamental period of 0.273 days and a visual light
range of 0.025 magnitudes.
    If we use the following formula derived for delta Scuti stars and Dwarf
Cepheids, (Fernie, 1964) :

               M_v = -1.5 - 2.50 log P + 3.50 (B-V)
and substitute into it our estimation of the period and the known B-V index
of 0.27, (Cousins and Stoy, 1962), we calculate M_v = 0.8 in agreement with
values stated earlier.

     The calculated metallicity (Delta m_1 = 0.026) suggests that S Eridani
is slightly metal-deficient, in accord with the majority of delta Scuti stars.
From the available date we conclude that S Eridani is probably a small
amplitude, longer period delta Scuti star. Derived absolute magnitude and
measured b-y (or B-V) colour index place this star at the high luminosity
end of the delta Scuti instability region.

     We would like to thank Mt. Stromlo and Siding Spring Observatories
for allocating us time on the 40 cm. telescope. This work was funded by
a Monash University special research grant.

     T. Moon is supported by a Commonwealth Government Postgraduate
Research Award.





       D.W. COATES, L. HALPRIN, T.T. MOON and R. THOMPSON.
         Department of Physics, Monash University,
           Clayton, Victoria 3168, Australia.





References:

Breger, M., 1979, Publs astr. Soc. Pacif., 91, 5-26. [BIBCODE 1979PASP...91....5B ]
Cousins, A.W.J., and Stoy, R.H., 1962, Roy. Obs. Bull. No. 64. [BIBCODE 1962smer.book.....C ]
Crawford, D.L., Barnes, J.V., and Golson, J.C., 1970, Astr. J., 75, 624-635. [BIBCODE 1970AJ.....75..624C ]
Eggen, O.J., 1979, Astrophys. J. Suppl., 41, 413-434. [BIBCODE 1979ApJS...41..413E ]
Fernie, J.D., 1964, Astrophys. J., 140, 1482-1493. [BIBCODE 1964ApJ...140.1482F ]
Leung, K.C., 1970, Astr. J., 75, 643-650. [BIBCODE 1970AJ.....75..643L ]
Millis, R.J., 1967, Ph.D.Thesis, University of Wisconsin, U.S.A.
Valtier, J.C., 1972, Astr. Astrophys. 16, 38-43. [BIBCODE 1972A&A....16...38V ]