COMMISSION 27 OF THE I. A. U.
                  INFORMATION BULLETIN ON VARIABLE STARS
                              Number 3590

                                                       Konkoly Observatory
                                                       Budapest
                                                       17 April 1991

                                                       HU ISSN 0374 - 0676


              THE PULSATION AMPLITUDE OF DELTA CETI

   Delta Ceti (HR 779, HD 16482) is a classical, no-frills, singly-periodic
Beta Cephei variable. Its one claim to fame rests on the well-observed secular 
changes in its period and velocity amplitude. These have been documented 
and discussed in detail by a number of authors (Lloyd & Pike,
1984; Jerzykiewicz et al., 1988). Lloyd & Pike emphasized the requirement
for consistent monitoring of this object with the hope that a substantial
database of observations, built up over many years, would aid in the 
understanding of this, one of the purest Delta Cephei variables.
   In this spirit, Kubiak and Seggewiss (1990) have recently reported an
analysis of observations from the 1986 observing window. From their 
spectroscopic data they present the striking result that the observed pulsation
velocity amplitude was nearly twice that previously observed and well in
excess of that expected from the ephemeris calculated by Lloyd & Pike.
The purpose of this note is to present the results of some spectroscopic 
observations taken by the authors rather fortuitously only some three months
before the Kubiak & Seggewiss data and to reiterate, before the next 
observing season, the need for further observations of this star. Full details
of our spectral observations together with some further photometry will be
published elsewhere. The spectra reported on here were obtained very early
in the 1986 season and as a result do not cover a full pulsation cycle. 
However, given the accuracy with which the period is known, the amplitude can
be estimated accurately enough from the available phase coverage.
   Our spectra were obtained at the Cassegrain focus of the Isaac Newton
Telescope at the Spanish observatory of El Roque de los Muchachos, La
Palma using a GEC CCD as detector and a grating giving a reciprocal
dispersion of 7.6 A/mm. The spectra were centred at 4580A so that each
exposure included nine sharp lines of Si III, O II and N II. The velocities
derived are given in Table I. The observed values have been corrected for
the Earth's motion but not for any instrumental zero point. The average
standard error of the mean velocity of each spectrum is 0.8 km/s. The
results are plotted in Figure 1 where they have been overlayed with a fitted
least-squares sinusoid of period 0.161137 days. The derived semi-amplitude
is K = 7.95 +- 0.10 km/s. This compares favourably with the value of K =
8.1 km/s predicted by the amplitude ephemeris of Lloyd & Pike (1984). The
time of velocity maximum is HJD 2446639.7815 +- .0002 which has an (O-C)
of +0.02 days from the Lloyd & Pike ephemeris and +0.01 days from the
more recent ephemeris of Kubiak & Seggewiss. We also note that our new
photometry is also consistent with previous photometry and ephemerides.


                 Table I

       Radial Velocities of HR 779


HJD+         km/s      HJD+         km/s
2446639                2446639

.64604       14.8      .70126        2.5
.64971       13.7      .70423        2.5
.65337       12.8      .70718        2.8
.65726       11.7      .71015        3.3
.66381        9.4      .71315        3.4
.66815        7.5      .71894        4.2
.67110        7.3      .72194        5.4
.67408        6.0      .72493        5.9
.67708        4.8      .72790        6.2
.68004        4.6      .73090        6.5
.68633        3.6      .73715        8.7
.68930        3.3      .74013       10.0
.69226        2.5      .74309       10.2
.69523        2.3      .74619       11.9
                       .74918       12.8


                             [FIGURE 1]


  If the result of Kubiak & Seggewiss represents a real amplitude increase
it must have been rather sudden. Its duration is unknown but the solution
to that may lie in some, as yet, unreduced data since we note that the
average time between acquisition and publication of new data on this star
is about 5 years! In any case it provides, yet again, an illustration of the
value of systematic monitoring and lest we forget..... another 5 years has
passed!


                                    C.D. Pike & C. Lloyd
                                    Rutherford Appleton Laboratory
                                    Chilton,
                                    Didcot,
                                    Oxon, OX11 0QX
                                    UK.



References :

Jerzykiewicz, M., Sterken, C. & Kubiak, M.,  A. & A. Suppl. 72, 449, 1988. [BIBCODE 1988A&AS...72..449J ]
Kubiak, M. & Seggewiss, W., Acta Astronomica, 40, 85, 1990. [BIBCODE 1990AcA....40...85K ]
Lloyd, C. & Pike, C.D. The Observatory, 104, 9, 1984. [BIBCODE 1984Obs...104....9L ]