COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3144 Konkoly Observatory Budapest 8 February 1988 HU ISSN 0374-0676 STROMGREN PHOTOMETRY OF UU Her: 1987 RESULTS UU Her is an F supergiant that seems to be the prototype of a class of evolved stars. All of them share peculiar photometric variability, spectral type and position very high above the galactic plane. Basic photometric features are the long periods, small amplitudes, and remarkable mode switching of two or more periods. In particular UU Her is known to switch between periods of approx. 45 and 72 days (Fernie 1986), in addition, Sasselov (1983) found the star to have a period of 80 days in 1978-1979. Within a program aimed to study the photometric and spectroscopic behaviour of these stars, we performed u,v,b,y photometry of UU Her, from June to November 1987 at the 91 cm Cassegrain telescope of the Catania Astrophysical Observatory with a photon-counting photoelectric photometer. The same set of standard Stromgren filters but different photomultipliers (EMI 6256/e and EMI 9658/Ra) were used throughout the observing period. Even if the EMI 9658/Ra has a sensitivity extended to the red, no difference due to the red leak has been found in the instrumental photometric system. Two main comparison stars BD +38 2798 ( V = 8.866, B-V = 0.417; Sasselov et al. 1987) and HR 6123 (V = 5.535, b-y = 0.112; Fernie 1986) have been used to carry out differential photometry. In addition ten u,v,b,y standard stars have been nightly observed to deduce b-y,m1,c1 indices. BD +38 2798 is named TZ Her as variable, but in the last General Catalogue of Variable Stars (Kholopov, 1985) is reported as non variable. No evidence of variability is seen from our observations displayed in Figure 1, where BD +38 2798 is plotted against HR 6123, in agreement with the photometry of Fernie (1986). The external error of our differential photometry estimated from the nightly averages of the differential observations between the two comparison stars, weighted by the dispersion around the mean of each night, turns out to be about 0.004 mag. Magnitude differences of UU Her with respect to HR 6123 in the y filter are shown in Figure 2 and a summary of the photometric results is given in Table I. The y filter light curve plotted in Figure 2. is characterized by two minima of different depth, so that UU Her in 1987 appears to show an RV Tau- like behaviour. The interval between the two minima is about 45 +/- 2 days [FIGURE 1] Figure 1 Differential magnitudes of our comparison stars. (BD +38 2798 - HR 6123) Error bar is the standard deviation of each night [FIGURE 2] Figure 2 Light curve of UU Her against HR 6123 Table I UU Her Stromgren Photometry in 1987 Julian Day Deltay b-y c1 m1 2440000.0+ 6999.425 3.478 0.429 0.788 0.129 7000.427 3.512 0.449 0.919 0.142 7001.412 3.517 0.417 0.919 0.141 7006.428 3.637 0.419 0.862 0.153 7014.481 3.660 0.310 0.863 0.247 7021.411 3.149 0.215 1.123 0.173 7029.396 3.353 0.414 1.003 0.078 7037.372 3.491 0.471 0.965 0.147 7039.363 3.484 0.446 0.982 0.190 7050.354 3.840 0.500 0.886 0.198 7059.320 3.863 0.459 0.824 0.124 7073.301 3.243 0.361 1.074 0.114 7118.275 3.489 0.346 0.899 0.241 and the difference in the depth in the y filter is about 0.2 mag. This light curve is in very good agreement with the one observed in 1961 (Fernie 1986). In 1984 and 1985 UU Her showed a fairly constant sinusoidal light curve with 71.3 day period (Sasselov et al., 1987), as that we have seen a new switch in the pulsation mode of this enigmatic star. G. UMANA 1; S. CATALANO 1, E. MARILLI 2, C. TRIGILIO 1 1 Institute of Astronomy, University of Catania 2 Catania Astrophysical Observatory Citta' Universitaria Viale A. Doria 6 95125 Catania, Italy References: Fernie J.D. 1986, Ap.J. 306, 642. [BIBCODE 1986ApJ...306..642F ] Kholopov P.N., 1985 General Catalogue of Variable Stars, Moscow, Nauka Sasselov D. 1983, IBVS, No. 2387. Sasselov D. Zsoldos E., Fernie J.D. and Arellano Ferro A. 1987, PASP 99, 967. [BIBCODE 1987PASP...99..967S ]