COMMISSION 27 OF THE I. A. U.
          INFORMATION BULLETIN ON VARIABLE STARS
                      Number 2846

                                           Konkoly Observatory
                                           Budapest
                                           13 January 1986

                                           HU ISSN 0374 - 0676



                THE VARIABILITY OF HD 17433


   We find that HD 17433 is variable; the timescale of variation is on the
order of a few weeks. Its light variation and color suggest that it is a
spotted variable, possibly an RS CVn-type star, with spots 800 K cooler than
the photosphere covering at least 6% of the surface.
   During a program to study changes in light curves of RS CVn variables, we
observed the K star HD 17433. As far as we can determine, it was suggested to
us as a possible spotted variable by Gregory W. Henry, who heard it has strong
Ca II H & K emission from Francis C. Fekel. Twenty observations were obtained
in the BVRI system (Cape R and I) on 10 nights in September 1984 with the No. 4
16-inch telescope at Kitt Peak National Observatory. The comparison star was HD
17395, and HD 17329 was observed as a check. These data have been deposited in
the IAU Commission 27 Archive for Unpublished Observations of Variables Stars
(Breger 1982) as file 147.
   HD 17433 was found to have the following colors, (B-V) = 0.97, (V-R)c =
0.55, and (V-I)c = 1.08, which are consistent with a middle K star. These
values, in fact, lie quite close to the expected colors of a K3-4 dwarf on the
standard relation of Bessell (1979). If the star is actually a giant, it would
be too blue in (B-V) by approx. 0.15 mag for the (B-V)--(V-I)c relation for giants.
The photometry is plotted in Figure 1. The star is clearly variable in both
light and color, the variation being at least consistent with a rotating
spotted star. Variations in V and (V-I) are greater than the 0.01 mag errors
assumed. The period is something on the order of three weeks, although we do
not seem to have observed a complete cycle and cannot give a definitive value.
Clearly, it cannot be shorter. Likewise, the timescale for a large spot to move
across the disk of a star, 0.4-0.55 rotational cycle, is not consistent with a
period much longer.
    Assuming a period of 20 days, we have calculated a rough spot model with
the techniques used in Poe and Eaton (1985). The calculated light variation is
shown by the solid curves in Figure 1. Values of parameters are as follows: i =
60deg (assumed), Teff = 4260 K, and Tspot approx.= 3500 K. Two spots were 
necessary to


                              [FIGURE 1]

 Figure 1. BVRI photometry of HD 17433. Nightly means are plotted
 against time. The star is obviously variable by ~ 0.1 mg in V and by
 ~ 0.03 mag in (V-I)c. The solid curves show the calculated light and
 color variation for a two-spot model having spots 800 K than the
 surrounding photosphere on a single star rotating once in 20 days.
 The fit with this model is good, although there seems to be a systematic 
 error in calculated (V-R)c color. If the spot were
 completely black, the variation in (V-I)c would be only 0.01 mag,
 considerably less than observed.



reproduce the asymmetry of the light variation. Both were taken to be at
latitude 40deg, their centers thus coming within 10deg of the center of the disk as
they transit, and they had angular radii of 10deg and 17deg, facing Earth at epochs
JD 2,445,969.0 and JD 2,445,975.0, respectively, in Figure 1. So the 
spots are
on the order of 800 K cooler than the photosphere and cover at least - 6% of
the surface of the star. Since we cannot be sure we detected the light curve's
minimum, the larger of the two spots could be even bigger than we have found.
Also, since the inclination of the rotation axis and the latitude of a spot are
not determined here, and since the latitudes of the two spots are likely
different, the coverage is probably greater than 6%.


                    JOEL A. EATON

                    Astronomy Department
                    Indiana University
                    Bloomington, IN 47405 U.S.A.


                    CLINT H. POE

                    Washburn Observatory
                    475 North Charter Street
                    Madison, WI 53706 U.S.A.



References:
Bessell, M. S. 1979, Pub. A.S.P., 91, 589. [BIBCODE 1979PASP...91..589B ]
Breger, M. 1982, Inf. Bull. Var. Stars, No. 2246.
Poe, C. H., and Eaton, J. A. 1985, Ap. J., 289, 644. [BIBCODE 1985ApJ...289..644P ]


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