Broad-band 20-micron photometry of 76 stars

Photometry in the 16- to 28- spectral band, made at Mauna Kea Observatory in 1971-1973, is presented for 76 stars, with a precision in most cases of 0.05 mag. The sample contains stars of classes B through M, as well as several carbon and S stars; most are Mira, semiregular, or irregular variables. The five primary standards and their 20- magnitudes are a Tau (-3.23), a Ori (-5.74), a Boo (-3.32), a Her (-4.26), and P Peg (-2.74). Observations of stars presumed to have black- body spectra, including a CMa, confirm the calibration adopted in a previous paper. An unexpected excess of 0.3 mag observed for a Lyr is tentatively attributed to chromospheric emission. Most stars were observed over several months, and 16 (all Mira variables) showed variations (up to 0.9 mag) at this wavelength. We present light curves for variables o Cet, R Cnc, S CrB, and R Aqr, and also for one Mira that did not vary, R Leo. A comparison of 20- magnitudes for these 76 stars with published data at 8.5 and 11 tL shows a strong correlation between brightness at 11 and 20 , with supergiants, including several Lc variables, having the largest excesses at both wavelengths. Subject headings:circumstellar shells - infrared sources - late-type stars - long-period variables - photometry