COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3077 Konkoly Observatory Budapest 27 August 1987 HU ISSN 0374-0676 IMPORTANT PARAMETERS OF FOUR SMALL-AMPLITUDE CEPHEIDS FROM uvby PHOTOMETRY The small amplitude cepheids HR8084 (DT Cyg), HR8157, HR7165 (FF Aql) and HR690, were observed in the uvby photometric system with a four-channel spectrograph photometer attached to the 1.5 m telescope at San Pedro Martir Observatory (Mexico), from October 19 to 25, 1986, The data were duely reduced to the standard system by means of standard stars selected from Crawford and Barnes (1970). The results are listed in Table I. The uncertainties are 0.010, 0.004, 0.006 and 0.015 in V, (b-y), m1 and c1 respectively. Table I, uvby observations of the program stars V b-y m1 c1 HJD (2440000.+) HR8084 5.875 .369 .222 .736 6724.722 5.909 .373 .228 .759 6729.663 HR8157 5.921 .334 .163 .696 6724.728 5.947 .337 .177 .700 6728.724 5.827 .305 .163 .813 6729.674 HR7165 5.547 .527 .224 .815 6728.588 HR690 6.234 .535 .189 .896 6723.830 6.229 .544 .192 .895 6724.839 6.271 .554 .209 .845 6726.875 6.321 .563 .218 .818 6727.858 6.335 .565 .222 .809 6728.832 ---- .560 .209 .826 6729.835 The light and color curves of HR690 are shown in Figure 1. To calculate atmospheric parameters, we have dereddened the mean values of the observations in Table I. The color excesses E(B-V), were calculated using the method of Dean at el. (1982) (see Arellano Ferro, 1984), and E(b- y)=0.73xE(B-V) was used (Crawford, 1975). The color excesses and the mean intrinsic photometric indices are given in Table II. [FIGURE 1] Figure 1.- Light and color variations of HR690 during October 1986. The observations have been phased with the ephemeris HJDmax 2444869.94+7.57E (Arellano Ferro 1984). Table II. Mean intrinsic photometric indices Star (b-y)0 m1_0 c1_0 E(B-V) HR8084 .349 .232 .744 0.021 HR8157 .285 .181 .728 0.055 HR7165 .396 .266 .789 0.180 HR690 .395 .257 .816 0.216 Figure 2 is a c1_0 vs. (b-y)0 diagram and it is sensitive to temperature and gravity. The positions of the program stars relative to the atmospheric models of Kurucz (1979) for [Fe/H]=0, indicate the Te and log g values listed in table III. [FIGURE 2] Figure 2.- Atmospheric models by Kurucz (1979) (solid line) for log g = 1, 2, and 3, and for Te=5500, 6000 and 6500 K, all for [Fe/H]=0. The positions of four small-amplitude cepheids are indicated (dots) and allow estimates of log g and Te for each star. Table III. Effective temperature and gravity Star log g Te(uvby) Te(H-alpha) HR8084 2.0 5900 6195 HR8157 2.5 6400 5950 HR7165 1.0 5700 5990 HR690 1.0 5700 5741 The uncertainties in E(B-V) are likely to be smaller than .03 (Dean et al., 1982). Such uncertainties and the fact that (b-y)0 in Table II has to be corrected to mean light, allowed us to estimate the uncertainties in Te and log g to be smaller than 500 K and .5 respectively. The calculated effective temperatures agree very well with those determined from H-alpha profile fits (Arellano Ferro, 1984). The absolute magnitudes Mv of the program stars were estimated using Antonello's (1985) calibration in terms of the bracket quantities [c1] and [m1]. With these values and Te from Table III we can estimate the stellar radii. Table IV. Absolute magnitudes and radii Star Mv Mv(W)* R/R0 R/R0(W)* HR8084 -2.55 -3.39 28 37+/-10 HR8157 -2.79 -3.14 26 34 HR7165 -3.57 -3.57 52 45.5+/-6 HR690 -3.71 -4.07 52 58+/-22+ * Mv (W) and R(W) are Wesselink values from Arellano Ferro (1984). + Radius from Burki and Benz (1982). Mv and radii values are listed in Table IV and are compared with the Wesselink values derived by Arellano Ferro (1984) and for HR690, with the radius found by Burki and Benz (1982). The agreement is satisfactory. We acknowledge Teresa Gomez, Guadalupe Salazar and Cristobal Jimenez for technical assistance. A. ARELLANO FERRO 1,* S. GIRIDHAR 2 L. PARRAO 3 1 Instituto de Astrofisica Optica y Electronica Apdo. Postal. 51 y 216, Puebla 72000, Mexico. 2 Indian Institute of Astrophysics Bangalore 560034, India. 3 Instituto de Astronomia, UNAM Apdo. Postal. 70-264, Mexico D.F. 04510, Mexico. * On leave from Instituto de Astronomia, UNAM. References: Antonello, E. 1985, IAU Symp. No. 111. D.S. Hayes, L.E. Pasinetti and A.G. Davis Philip, eds. (Dordrecht: Reidel), p.491. [BIBCODE 1985IAUS..111..491A ] Arellano Ferro, A. 1984, M.N.R.A.S., 209, 481. [BIBCODE 1984MNRAS.209..481A ] Burki, G., and Benz, W. 1982, Astr. Ap., 115, 30. [BIBCODE 1982A&A...115...30B ] Crawford, D.L. 1975, A.J., 80, 955. [BIBCODE 1975AJ.....80..955C ] Crawford, D.L. and Barnes, T.G. 1970, A.J., 75, 978. [BIBCODE 1970AJ.....75..978C ] Dean, J.F., Warren, P.R. and Cousins, A.W.J. 1978, M.N.R.A.S., 183,569 [BIBCODE 1978MNRAS.183..569D ] Kurucz, R.L. 1979, Ap. J. Suppl., 40, 1. [BIBCODE 1979ApJS...40....1K ]