COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 1914

                                  Konkoly Observatory
                                  Budapest
                                  1981 January 27

                                  HU ISSN 0374-0676



        THE PERIOD OF THE Bp STAR HD 213918



   HD 213918 (BD +38d4801), m_v =8.715, is a very hot,                
Si lambda4200 star (Bertaud, Floquet 1974).  Its projected
rotational velocity is 80 km/s according to Wolff (1980).
It has a rather deep lambda5200 depression, since its peculiarity
index Z is -0.048, leading to a photometric estimate of the
mean surface field H_S (Z, X) = 4.0 kG (Cramer and Maeder
1980a, b).  It is a visual double star (ADS 16064) with
1.8" separation and visual magnitudes 8.7 and 13.2.   So
the companion has a negligible influence on the colours and
amplitude of variations.
   Furthermore, HD 213918 is a member of the association
Lacerta OB1 and is relatively young (1.26 10^7 yrs after Abt
(1979)).   This fact, together with the high estimated field
(4.0 kG is actually a lower boundary, because the relation
between Z and H_S is not bijective) makes such an object 
interesting.
   It was observed between 26 September and 4 October 1980
from the Gornergrat Observatory with the 1m telescope, in the
Geneva photometric system.  Eleven differential measures
have been obtained, with HD 213801 and HD 214243 as comparison
stars; these proved to be constant over the observing time
span, while HD 213918 shows variations of about 0.07m through
the V filter (the measures of the other magnitudes have not
yet been reduced).    A period P=1.43 +- 0.03 days was derived
using the Theta_1 test proposed by Renson (1978), see Fig. 1.

                      [FIGURE 1]
                      [FIGURE 2]

The V magnitudes relative to the first comparison star
(HD 213801) are shown in Fig. 2 as a function of the phase,
according to the ephemeris

        J.D. (max. light) = 2444509.32 + 1.43d E

Differential extinction was taken into account.
If a radius R = 3 R_Sun is assumed, then the real rotational
velocity may be estimated from the relation V = 50.6 R/P
= 106 km/s. This is about half the value admitted for field
late-B stars (Slettebak 1954).    From Vsini = 80 km/s the
inclination is 49d.





                      P. NORTH

                   Institut d'Astronomie de l'Universite
                   de Lausanne et
                   Observatoire de Geneve
                   CH-1290 Chavannes-des-Bois
                   Switzerland




REFERENCES


Abt, H.A.: 1979, Astrophys. J. 230, 485 [BIBCODE 1979ApJ...230..485A ]
Bertaud, Ch., Floquet, M.: 1974, Astron. Astrophys. Suppl.
   16, 71 [BIBCODE 1974A&AS...16...71B ]
Cramer, N., Maeder, A.:  1980a, Astron. Astrophys. 88, 135 [BIBCODE 1980A&A....88..135C ]
                         1980b, Astron. Astrophys. Suppl. 41, 111 [BIBCODE 1980A&AS...41..111C ]
Renson, P.: 1978, Astron. Astrophys. 63, 125 [BIBCODE 1978A&A....63..125R ]
Slettebak, A.: 1954, Astrophys. J. 119, 146 [BIBCODE 1954ApJ...119..146S ]
Wolff, S.C.: 1980, to be published in Astron. Astrophys. [BIBCODE 1981ApJ...244..221W ]