The Hipparcos mission discovered a few dozen M giant stars with periods P shorter than 10d. Similar stars may be found in other large data bases of new variables (e.g., OGLE). The three possible sources of the magnitude variations - pulsation, starspots and ellipsoidal deformation - are discussed in general terms. The parallaxes and V-I colour indices are used to calculate radii and temperatures for all M giant variables with P<100d. Masses are estimated from the positions of the stars in a Hertzsprung-Russell (HR) diagram, using evolutionary tracks. Using these data, it is shown that starspots can be ruled out as a variability mechanism in almost all cases, and ellipsoidal variations in about half of the stars. Pulsation in very high-overtone modes appears to be the only viable explanation for the stars with P<10d. Many of the stars may be multiperiodic. IRAS data are used to deduce information about reddening and circumstellar dust. The apparently low level of mass-loss, as well as the kinematics and the spatial distribution of the stars, indicates that they are from a relatively young (i.e., thin disc) giant star population.