COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                     Number 1096

                                 Konkoly Observatory
                                 Budapest
                                 1976 February 7



      ON PERIOD-SPECTRUM RELATION FOR DELTA SCUTI STARS
                   AND DWARF CEPHEIDS


    To investigate the question on possible differences between
Delta Scuti variables and Dwarf Cepheids we show below (Fig. 1) the
period-mean spectral type (hydrogen lines) diagram for these stars.
The data are taken from different sources. Crosses and circled
crosses denote Delta Scuti stars in galactic field and in open clusters, 
respectively (BS 1611, maybe, has P>0.2d); open circles denote
Dwarf Cepheids, for which extremal limits of spectral type variations 
are designed by vertical lines and their short GCVS-names are
given, too (SX Phe, CY Aqr, ZZ Mic, DY Peg, EH Lib, AI Vel,
V 703 Sco, AD CMi, RS Gru, DY Her, V 567 Oph, VZ Cnc, and
BS Aqr in order of their period lengthes). On this diagram only
those stars are shown for which two-dimensional MK-types are available. 
Two stars in NGC 2264 are ignored.

                       [FIGURE 1]

    There exists a real difference between Delta Scuti stars
(either in galactic field or in clusters) and Dwarf Cepheids with
periods shorter than 0d1: Dwarf Cepheids have systematically 
earlier spectral types or lie just on the high border line of Delta
Scuti-region. One can hardly confirm the same for stars with
longer periods. A similar picture can be seen on beta_1 - P diagram, as
well (Jones, 1973, Fig. 7, p. 500).         
    Only the Delta Scuti stars having periods shorter than 0.1d
agree with extrapolated P-Sp relation for Cepheids (straight line
on Fig 1). This confirms the idea that Delta Scuti stars (but only
with F<0.1d) are shortperiodic analogues of Cepheids.
    Different "spectral pictures" for Delta Scuti stars and Dwarf
Cepheids with different periods are in accordance with their other
features (Frolov, 1974), in particular with DeltaV versus log P diagram
(Fig. 2) for all ultrashortperiodic A-F stars without any splitting
on types of variability (crosses denote photoelectric measurements).
Fig. 2 was already discussed earlier by the author (Frolov, 1974).

                    [FIGURE 2]


                                     M.S. FROLOV
                                 Astronomical Council
                            of the USSR Academy of Sciences



References:

Frolov, M.S. 1974, Sci.Informations of USSR Astron.Council,
        No. 33, 29.
Jones, D.H.P., 1973, Astrophys.J. Suppl. 25, 487. [BIBCODE 1973ApJS...25..487J ]