COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                   Number 3155

                                Konkoly Observatory
                                Budapest
                                15 March 1988

                                HU ISSN 0374 - 0676





           THE PERIOD OF TWO Bp Si Mg STARS:
              HD 60431 AND CoD -51 3378



   The Si Mg peculiarity type is extremely rare among the hot Ap
stars with enhanced silicon lines. To our knowledge, there are
only three of them: HD 3473, HD 60431 and CoD -51 3378. The
latter two are in the southern celestial hemisphere and were
monitored with the Swiss telescope at the European Southern
Observatory, La Silla (Chile), using the Geneva photometry. HD
60431 was observed at the beginning of 1987, while CoD -51 3378
(classified Si Mg by Bidelman, 1985) was measured in January 1988.
   Both stars show a fairly large amplitude in the [U] band,
reaching almost 0.1 m peak-to-peak, while the amplitude in the V
band hardly reaches 0.04 m. All colours vary in phase.
   The period of HD 60431 is very short and certainly holds the
record among the classical magnetic Ap stars (Preston's CP2
category): we have obtained
         P = 0.475518 +/- 0.000059 d.!

   This is shorter than the shortest periods known to date, such
as 0.5207 (HD 124224, Si) or 0.51747 (HD 164429, Si Cr Sr). The
only star which could have a similar, or even shorter. period is
HD 177517 (Hg Si), but the published periods are ambiguous and
uncertain (P=0.4877 or 0.33772 d., Manfroid and Mathys, 1985).


                      [FIGURE 1]
                      [FIGURE 2]


Applying the oblique rotator model, i.e. the well-known formula
v_eq =50.6 R/P, such a short period implies an equatorial velocity
v_eq =245 km s^-1 if we assume R = 2.30 R_sol. The latter radius holds
for a 4.5 M_sol star lying on the ZAMS (the mass is deduced from the
calibrated X and Y parameters of the Geneva photometry), so it
must be considered rather as a lower boundary. Therefore, the 245
kmsE-1 value of the equatorial velocity is probably a lower limit
too. Such a rapid rotational velocity is more typical of the
normal stars than of the Ap stars, and may be a challenge to the
theory of radiative diffusion, the most promising one for
explaining the Ap stars' abundance patterns. Indeed, rapid
rotation induces meridional circulation, which will tend to
counteract the diffusion processes. It would be most interesting
to determine the magnetic field, which is an essential ingredient
in the diffusion processes, among the Si stars (Megessier, 1984).
   CoD -51 3378, on the contrary, has a period
             P= 1.282 +/- 0.006
which is quite common, since it is close to the maximum of the
period distribution of the young Si stars (North, 1987).
   Fig. 1 shows the lightcurves of HD 60431 for the [U] and V
bands. The magnitudes are differential ones, relative to HD 61031.
   Similarly, Fig. 2 shows the lightcurves of CoD -51 3378. Here
the differential magnitudes are relative to HD 76805.





            P. NORTH, J. BABEL and T. LANZ
            Institut d'astronomie de
            l'Universite de Lausanne et
            Observatoire de Geneve
            CH-1290 CHAVANNES-DES-BOIS
            Switzerland


References:
Bidelman, W.P.: 1985, Astron. J. 90, 341 [BIBCODE 1985AJ.....90..341B ]
Manfroid, J. and Mathys, G.: 1985, Astron. Astrophys. Suppl. Ser.
 59, 429 [BIBCODE 1985A&AS...59..429M ]
Megessier, C.: 1984, Astron. Astrophys. 138, 267 [BIBCODE 1984A&A...138..267M ]
North, P.: 1987, Astron. Astrophys. Suppl. Ser. 69, 371 59 [BIBCODE 1987A&AS...69..371N ]