COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2512 Konkoly Observatory Budapest 2 May 1984 HU ISSN 0374-0676 INFRARED OBSERVATIONS OF EPSILON AURIGAE In the night of Dec. 14, 1983, epsilon Aur and six other stars were observed with TIRGO, a 1.5-m telescope installed on the Gornergrat (Switzerland). A circular variable filter and an InSb photocell permit observations in 37 steps between lambda = 2.84 and 4.20 microm. After having allowed for the transmittance of the filters and assuming a constant quantum efficiency of the cell in this range of wavelengths, the observations of the six stars were then compared with Planckians calculated according to a colour-Teff calibration (Bohm-Vitense 1981), thus a normalized mean atmospheric transmittance curve was deduced (Figure 1). For the reduction to an international system, available L magnitudes were collected from the literature and the following reduction [FIGURE 1] [FIGURE 2] formula was used 8.26-2.5 log l, l corresponds to the counts corrected for the transmittance of filters and atmosphere. All stars were observed around their upper culmination. The arithmetical mean of all observations for each star, between 2.91 and 4.17 microm (central wavelength: 3.54 microm) are given in Table I, whereas the single reduced points are plotted in Figure 1. When the same procedure was applied to epsilon Aur, the observed values did not fit any Planck function for temperatures corresponding to its spectral type. After some attempts, two Planckians were adopted. The first was related to the spectral type of epsilon Aur and the other remained unchanged around a temperature of 700K Table I L magnitudes Star Sp.type this other values References paper alpha Cet M1.5III -1.85 -1.74;-1.78;-1.87 Johnson et al. 1966, Lee 1970, Glass,1974 alpha Per F5Ib +0.37 +0.48 Low and Mitchell,1965, Johnson et al.1966 rho Gem F0V +3.27 upsilon UMa F0IV +2.99 +2.98 Glass 1975 alpha UMa K0III -0.78 -0.78 Johnson et al.1966 gamma UMa A0V +2.44 +2.4 Woolf et al.1970 epsilon Aur F2Ib +1.95 +1.25;+1.23 Low and Mitchell,1965 Johnson et al. 1966 even changing the stellar temperature between 7200K and 6740K. The fit was satisfactorily good and the mean arithmetical magnitude 1.95 resulted, with a fitting error of +-0.04. Since the magnitude out eclipse is 1.25 (see Table I), the decrease within eclipse would be 0.7 magnitudes, which corresponds to 52% of the total magnitude of epsilon Aur. But, according to our model, the star still contributes for about 80% to the luminosity during eclipse and the remaining flux should come from the eclipsing body. We wish to thank for the hospitality the Centro per l'Astronomia Infrarossa and in particular Miss Leslie Hunt for the assistance during the observations. C. BOEHM B. CESTER Osservatorio Astronomico Universita degli Studi Trieste, Italy Trieste, Italy References: Bohm-Vitense, E., 1981, Ann.Rev.Astron.Astrophys. 19, 295 [BIBCODE 1981ARA&A..19..295B ] Glass, I.S., 1974, Mon.Not.R.Astr.Soc. South Africa 33, 53 [BIBCODE 1974MNSSA..33...53G ] Glass, I.S., 1975, Mon.Not.Roy.Astron.Soc. 171, 19P [BIBCODE 1975MNRAS.171P..19G ] Johnson, H.L., Mitchell, R.I., Iriarte, B., Wisniewski, W.Z., 1966, Lunar and Planetary Lab. 4, 99, Communic. No. 63 [BIBCODE 1966CoLPL...4...99J ] Lee, Th.A., 1970 Astrophys.J. 162, 217 [BIBCODE 1970ApJ...162..217L ] Low, F.J., Mitchell, R.I., 1965, Astrophys.J. 141, 327 [BIBCODE 1965ApJ...141..327L ] Woolf, H.J., Stein, W.A., Strittmatter, P.A., 1970, Astron. and Astrophys. 9, 252 [BIBCODE 1970A&A.....9..252W ]