Microvariability and fast variability of stars. II. The semiregular red giants L2 Pup, 2 Cen, η Gem, and σ Lib

This is a study of the photometric magnitudes Vt and Bt of four semiregular red giants of spectral classes M3-M6 III, L₂ Pup, 2 Cen, η Gem, and σ Lib, from the Hipparcos catalog to search for microvariability and rapid variations in their emission. It is shown that low amplitude variability shows up in the form of rapid changes in the brightness minimum of the main cycle. The magnitudes of 2 Cen and σ Lib changed by 0^m.10-0^m.15 over 15-20 min. These variations were repeated over time intervals that were multiples of 150-180 min. The spread in the magnitudes of L₂ Pup was 0^m.02-0^m.06 over days. Rapid bursts with brightness increases of up to ~0^m.1 within seconds, over time intervals of <21 min, were observed in η Gem. These bursts could, presumably, be induced by a hypothetical nearby satellite upon its closest approach to the star. Then the orbital period of the satellite is equal to the period of the star's pulsations. The existence of such a satellite requires further confirmation.