COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 1718 Konkoly Observatory Budapest 1979 December 18 RADIAL-VELOCITY VARIATIONS OF DY HERCULIS Joy (1950) was the first to measure the radial velocity of DY Her. From three spectra he obtained a mean velocity of gamma=-50 kms^-1 and estimated a radial velocity amplitude of 70 kms^-1. Later on Bonsack (1957) obtained complete radial velocity curves of some of the short-period variable stars. DY Her was also included in his programme in which he made 11 spectra for radial velocity measurements of this star on four nights in 1955 and 1956. The radial velocity curve of DY Her determined by him differed in shape from that of other short-period variables. Using Bonsack's radial velocity curve Hardie and Lott (1961) have already attempted to determine the radius of this star by Baade's and Wesselink's method. Their results, however, were very much affected by the lack of sufficient precision of photoelectric photometry and radial velocity measurements. It is thus hardly surprising that they failed to get a reliable radius. Radial velocity measurements were secured from a single- trail spectrogram of DY Her by McNamara (1978), Utilizing the high precision uvbybeta photometry of Breger et al. (1978) he found a radius of 2.7 R_sun using Wesselink's method. This value is considerably smaller than the value one would expect from McNamara and Feltz's (1976) expression for the dependence of the radius on the period derived from surface gravity measurements. McNamara (1978) remarks that "increasing the velocity amplitude of DY Her to 35 kms^-1 would lead to a Wesselink radius near to this (expected) value". Comparing the total velocity amplitude 2k=30kms^-1 of DY Her obtained by McNamara (1978) with the total velocity amplitudes of other dwarf cepheids (e.q: 2k=45.5 kms^-1 for EH Lib and 44 kms^-1 for SZ Lyn given by McNamara and Feltz, 1976) we suspect that it has been underestimated. In order to clarify this problem it seems to be worth publishing our radial velocity measurements. In 1971 five spectrograms (at a reciprocal dispersion of 60 Amm^-1) were secured with the Cassegrain spectrograph of the 72 inch telescope of the Dominion Astrophysical Observatory on baked IIa-O plates on two nights. The plates were measured on the DAO Arcturus measuring machine. Spectrograms of a number of standard-velocity stars were also secured on both nights and measured to check on possible systematic errors. None were found. In Table I we list the plate number, the date and Julian day of each spectrogram, the number of lines measured, the radial velocity and the phase. Table I Radial velocities of DY Her Plate Date Heliocentric No. of R.V. phase No. J.D. lines kms^-1 71321 1971.06.30 2441132.757 19 -30.9+-3.8 0.7850 71322 .800 16 -60.6+-4.7 0.0743 71323 .847 13 -49.4+-3.4 0.3906 71415 1971.07.22 2441154.750 12 -36.0+-4.6 0.7554 71416 .806 13 -65.8+-4.6 0.1322 The phases were calculated using our elements with a quadratic term (Szeidl and Mahdy, 1979): Max.hel.=J.D.2433439.4865+0.148631349d*E- -18.04*10^-13d*E^2. [FIGURE 1] In Figure 1 we have plotted Bonsack's, McNamara's and our radial velocity measurements against phase; a free-hand velocity curve has been drawn through our observations. A mean velocity of about gamma=-47 kms^-1 was found from our velocity data. (Bonsack obtained a mean value of -45.8 kms^-1; McNamara gave the mean velocity as gamma=-49 kms^-1) Relying on our own data we fix the maximum and minimum velocities of the DY Her velocity curve at about -32 kms^-1 and -66 kms^-1, respectively, which yield a velocity amplitude of 34 kms^-1. Although our results confirm that the radial velocity amplitude of DY Her is smaller than the amplitudes of EH Lib and SZ Lyn, these results nevertheless suggest a larger Wesselink radius for DY Her than the value obtained by McNamara (1978). New radial velocity measurements of DY Her with higher dispersion and higher time resolution are needed to obtain a more reliable radius of this star. B. SZEIDL Konkoly Observatory Budapest, Hungary References: Bonsack, W., 1957, Astrophys. J. 126, 291 [BIBCODE 1957ApJ...126..291B ] Breger, M., Campos, A.J. and Roby, S.W., 1978, Publ. Astr. Soc. Pacific 90, 754 [BIBCODE 1978PASP...90..754B ] Hardie, R.H. and Lott, S.H., 1961, Astrophys.J. 133, 71 [BIBCODE 1961ApJ...133...71H ] Joy, A.H., 1950, Publ. Astr. Soc. Pacific 62, 60 [BIBCODE 1950PASP...62...60J ] McNamara, D.H., 1978, Publ. Astr. Soc. Pacific, 90, 759 [BIBCODE 1978PASP...90..759M ] McNamara, D.H. and Feltz, A.Jr., 1976, Publ. Astr. Soc. Pacific 88, 164 [BIBCODE 1976PASP...88..164M ] Szeidl, B. and Mahdy, H.A., 1979, Mitteilungen Sternw. Budapest, Nr. 75 [BIBCODE 1981CoKon..75....1S ]