COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 1968

                                  Konkoly Observatory
                                  Budapest
                                  1981 May 19

                                  HU ISSN 0374-0676



   SEVEN-COLOUR-PHOTOMETRY OF THE SILICON VARIABLE HD 37808 +)


Very recently Renson and Manfroid (1981) published uvby lightcurves 
of HD 37808 based on a period of 1.099 days determined
from a sample of 33 measurements which were obtained in Nov. 1977.
I have also obtained 10 measurements on 10 consecutive nights in
Feb,1973 at the ESO 1m-telescope using standard uvby and in addition
H-beta-wide and the filters g_1 and g_2 introduced by Maitzen (1976)
for measuring the strength of the absorption feature around 5200 A.
The first impression of this series of measurements was a variation 
with a period of about 10 days. As such a period would be
extraordinarily long for a Silicon star the associated period
close to one day seemed more plausible.
Unfortunately the time gap to the observations of Renson and
Manfroid is too large and hence the refining of the period 1.099
days is not yet possible until other measurements become available.
Fig.1 shows in the upper part the lightcurves reduced with the
period 1.099 days and JD 2441728.18 as zero-point for the phases.
Differential magnitudes were obtained using HD 37635 as comparison
star which was also one of Renson and Manfroid's comparison stars.
In the lower part of Fig. 1 the run with wavelength of the maxima
and minima of the lightcurves is displayed. One easily recognizes
the depressions at g_2(5240A) and at v (4100A) which are typical
for Ap stars.
The curve of the minima shows smaller depressions than that of
the maxima. This is well explained by lower absorption both in
the ultraviolet and in the visual depression regions. The former
results in a smaller backwarming effect. Both effects modulate
the depth of the depression features.
In general, we learn from the lower part of Fig. 1 that the slope
of the Paschen continuum of HD 37808 is very similar (albeit a
bit steeper) to the Paschen slope of the comparison star HD 37635.


+)Based on observations collected at the European Southern Observatory 
  (ESO), La Silla (Chile).



TABLE I    Journal of observations:         HD 37808 minus HD 37635.
J.D.hel    u        v        b          Hbetaw   g_1       g_2       y
2440000+
1728.564   -0.054   -0.035   -0.039   -0.031   -0.029   -0.005   -0.024
1729.557       71       41       51       44       40       15       36
1730.541       92       48       58       50       37       12       37
1731.538       99       56       65       62       50       22       42
1732.584      105       64       69       62       53       22       45
1733.545       90       56       58       52       48       19       35
1734.530       81       44       57       48       38       09       32
1735.557       74       44       49       44       38       11       31
1736.566       64       34       40       35       31       11       23
1737.557   -0.066   -0.029   -0.036   -0.037   -0.031   -0.002   -0.021

                        [FIGURE 1]

 FIG. 1: Upper part: Lightcurves in the sense HD 37635 minus HD 37808
 in uvbbetag_1g_2y. Phases were obtained using the elements:

                    Max(u) = JD 2441728.18 + 1.099 E.

 Lower part: Maxima (open circles) and minima (stars) of the lightcurves
 in the upper part versus wavelength.


This is in accordance with similarity in the uvby (see Hauck,
Mermilliod, 1980) and UBV (see Blanco et al., 1970) values of
both stars.
As for other Ap-stars there is a discrepancy in spectral types:
Cowley (1972) classifies HD 37808 as B9.5 IIIp Si 4200. while
Lesh (1968) gives B7V for HD 37635 for which the HD type is B5.
From the uvby calibration of B type stars (Crawford, 1978) one
gets B5-6V for the Ap star and B6V for the comparison star.
Thus the Ap line criteria yield a significantly cooler spectral
type than the continuum flux distribution of the Ap star. In
addition, the relatively weak hydrogen lines were explained by
higher luminosity compensating the later type of the spectral
classification.


                      HANS M. MAITZEN

                      Institut fur Astronomie der
                      Universitat Wien
                      Turkenschanzstr. 17
                      A-1180 WIEN/Austria





References:

Blanco, V.M., Demers, S., Douglass, G.G., Fitzgerald, M.P.: 1970,
     Publ. US Naval Obs. Second Ser. Vol. 21 [BIBCODE 1968PUSNO..21....1B ]
Cowley A.: 1972, Astron. J. 77, 750 [BIBCODE 1972AJ.....77..750C ]
Crawford, D.L.: 1978, Astron. J. 83, 48 [BIBCODE 1978AJ.....83...48C ]
Hauck, M., Mermilliod, M.: 1980, Astron.Astrophys. Suppl. 40, 1. [BIBCODE 1980A&AS...40....1H ]
Lesh, J.R.: 1968, Astrophys.J. Suppl. 17, 371 [BIBCODE 1968ApJS...17..371L ]
Maitzen, H.M.: 1976, Astron.Astrophys. 51, 223 [BIBCODE 1976A&A....51..223M ]
Renson. P., Manfroid, J.: 1981, Astron.Astrophys.Suppl. 44, 23. [BIBCODE 1981A&AS...44...23R ]