COMMISSION 27 OF THE I. A. U.
       INFORMATION BULLETIN ON VARIABLE STARS
                   Number 2323

                                 Konkoly Observatory
                                 Budapest
                                 26 April 1983

                                 HU ISSN 0374 - 0676


          OBSERVATIONS OF THE RS CVn STAR HD 26337
      WITH THE INTERNATIONAL ULTRAVIOLET EXPLORER SATELLITE



  HD 26337 (m_V = +7.05; G5IV) was recently discovered to be a single-line
spectroscopic binary and a low amplitude light variable by Fekel et al. (1982).
This star had been previously listed by Bidelman and MacConnell (1973) as having
noticeable CaII H + K emission. Fekel et al. confirm this result and report
moderately strong Ca II H + K emission and rotationally broadened absorption
features indicating a v sin i of about 40-45 km/s. A preliminary analysis of
radial velocity observations obtained on 10 nights by Fekel et al. indicates
an orbital period of 2.04414d+-0.00047d with a velocity amplitude of about 5
km/s. Their photoelectric photometry, obtained chiefly in the V-bandpass, on
4 nights during 1979/80 and 24 nights during 1980/81 reveal the star to be a
light variable having a quasisinusoidal light curve with an amplitude of about
0.16m. In addition, the photometric period, presumably marking stellar evolution, 
appears to be slightly shorter (by 0.3%) than the orbital period, where
the light elements given by Fekel et al. are:

            t_min = JD 2444635.65 + 2.038d E   (1)

The strong Ca II H + K emission, the mid-G spectral type, and the 2.04d 
quasisinusoidal light variation with a period nearly synchronous with its orbital
period reported for HD 26337 all are characteristics of RS CVn variables as
defined by Hall (1976).
  HD 26337 was observed on 4 days between 1982 December and 1983 March with
the International Ultraviolet Explorer (IUE) satellite in order to obtain 
ultraviolet spectra in the vicinity of the chromospheric Mg II h + k features at
lambda2800. A comprehensive description of the IUE satellite and its scientific
instrumentation is given by Boggess et al. (1978). During our observations of
the star, the Fine Error Sensor (FES) on board the satellite was used as a
photometer to measure the optical brightness of the star. Because the absolute 
sensitivity of the FES can vary up to ~0.10m over a day, we measured the
brightness of HD 26337 with respect to the nearby comparison star 37 Eri (HD
26409). This is the same comparison star used in the ground-based photometry
for which Nicolet (1978) gives V = +5.44m and B-V = +0.94m. The comparison star
was observed before and after the variable star with both stars placed at the
same reference position of the detector. The relative brightness of each star
was obtained by averaging the count rates from multiple scans of the image
dissector of the FES where the precision of the relative magnitudes is about
+-0.01m. The counts were converted to V-magnitudes of the UBV system by using
the calibration of Holm and Crabb (1979). In making differential measures,
the transformation equation simplifies to:
  
         Delta V_1-2 = -2.5 log(C_1/C_2) - 0.24 Delta(B-V)_1-2      (2)

where C_1, C_2 are the FES count rates for stars 1 and 2, and Delta(B-V)_1-2 is the
difference in the (B-V) colors of the two stars. Fekel et al. report a mean
color difference between the variable and comparison stars of Delta(B-V)_v-c=-0.27m,
and we adopted that value in the reductions. The similarity of the B-V indices
of the comparison and variable stars diminishes the effect of the color term
of the transformation equation and increases the accuracy of the DeltaV 
determination.

  The differential V magnitudes, in the sense variable minus comparison,
are plotted against the photometric phase in Fig. 1. The 1979/80 and 1980/81
DeltaV measures of HD 26337, reported by Fekel et al., also are plotted in the
figure. As shown, the mean brightness of the star has decreased from DeltaV=+1.62
during 1980/81 to DeltaV=+1.81 during 1982/83. Furthermore, there is an 
indication from the 1979/80 data that the star was brighter during that year with
an estimated mean brightness of DeltaV ~ +1.57m.
  In addition to a decrease in the mean brightness of the star with time,
it also appears that the amplitude of the light curve has lessened from about
0.16m in 1980/81 to about 0.10m in 1982/83. The phase of light minimum is not
well determined by the 1982/83 observations, but from the few observations
obtained, it appears to be close to that predicted using Eq. (1).
  As in the case of other RS CVn variables, if the light variation of HD
26337 arises from the presence of a nonuniform longitudinal distribution of
surface inhomogeneities (=starspots) on a rotating star, then the observed
changes in the light curves with time imply substantial changes in the relative 
spot distribution as well as in the total spotted area. To a first 
approximation, the observed decrease in the mean light of the star implies an
overall increase in the total area of starspots that are in view. The decrease
in light amplitude coupled with the decrease in mean brightness indicates an
increase in the spotted area along with a net movement of the spot centers
toward higher latitudes (i.e. toward the rotational pole of the star that is
in view). Similar light curve changes were reported by Dorren and Guinan
(1482) for the bright RS CVn variable V 711 Tau (=HR 1099) and by Guinan et
al. (1982) for UX Ari.

                           [FIGURE 1]

   Figure 1. The differential V-magnitudes of HD 26337 (measured relative
   to 37 Eri) plotted against photometric phase, where the phases
   were computed from Eq. (1). The 1979/80 and 1980/81 observations
   are from Fekel et al. The 1982/83 observations were obtained with the
   Fine Error Sensor (FES) on board the IUE satellite.

  Preliminary reductions of the ultraviolet spectra of HD 26337 reveal 
moderately strong and variable MgII h+k lambda2800 emission, indicative of a 
chromospherically active star. We plan to report on the ultraviolet observations in
a subsequent paper. More photometry of HD 26337 is certainly desirable to 
define the nature of the long-term changes in the light curve suggested by the
existing data.
  We are happy to acknowledge support for this work from NASA research
grant NAG5-87.

          


               S.L. BALIUNAS ^a, W.P. BLAIR ^a and E.F. GUINAN ^a
               Harvard-Smithsonian Center for Astrophysics
               60 Garden Street, Cambridge, MA 02146, USA

a) Guest Observer, International Ultraviolet Explorer Observatory,
   NASA Goddard Space Flight Center, Greenbelt, MD.





References:

Bidelman, W.P. and MacConnell, D.J. 1973, Astron. J., 78, 687. [BIBCODE 1973AJ.....78..687B ]
Boggess, A., and 33 other authors, 1978, Nature, 275, 377. [BIBCODE 1978Natur.275..377B ]
Dorren, J.D. and Guinan, E.F. 1982, Astrophys. J., 252, 296. [BIBCODE 1982ApJ...252..296D ]
Fekel, F.C., Hall, D.S., Henry, G.W., Landis, H.J., and Renner, T.R.  1982,
    I.B.V.S., No. 2110.
Guinan, E.F., McCook, G.P., Fragola, J.L., O'Donnell, W.C., and Weisenberger,
    A.G. 1981, Publ. Astron. Soc. Pac., 93, 495. [BIBCODE 1981PASP...93..495G ]
Hall, D.S. 1976, in Multiple Periodic Variable Stars, IAU Colloq. 29, W.S.
    Fitch ed.(Dordrecht: Reidel Publishing Co.), Vol. I, p. 287. [BIBCODE 1976ASSL...60..287H ]
Holm, A.V., and Crabb, W.G. 1979, IUE NASA Newsletter, 7, 40.
Nicolet, B. 1978, Astron. Astrophys. Suppl., 34, 1. [BIBCODE 1978A&AS...34....1N ]