red-giant branch before the helium flash is more difficult to measure directly. The current mass of Cepheid variables such as δ Cephei can be measured accurately because there are either binaries or pulsating stars. When compared with evolutionary models, such stars appear to have lost around 20% of their mass, much of it during the blue loop and especially during pulsations on the instability strip. Variability Some red giants are large amplitude variables. Many of the earliest-known variable stars are Mira variables with regular periods and amplitudes of several magnitudes, semiregular variables with less obvious periods or multiple periods and slightly lower amplitudes, and slow irregular variables with no obvious period. These have long been considered to be asymptotic giant branch (AGB) stars or supergiants and the red-giant branch (RGB) stars themselves were not generally considered to be variable. A few apparent exceptions were considered to be low-luminosity AGB stars. Studies in the late 20th century began to show that all giants of class M were variable with amplitudes of 10 milli-magnitudes of more, and that late-K-class giants were also likely to be variable with smaller amplitudes. Such variable stars were amongst the more luminous red giants, close to the tip of the RGB, but it was difficult to argue that they were all actually AGB stars. The stars showed a period amplitude relationship with larger-amplitude variables pulsating more slowly. Microlensing surveys in the 21st century have provided extremely accurate photometry of thousands of stars over many years. This has allowed for the discovery of many new variable stars, often of very small amplitudes. Multiple period-luminosity relationships have been discovered, grouped into regions with ridges of closely spaced parallel relationships. Some of these correspond to the known Miras and semi-regulars, but an additional class of variable star has been defined: OGLE Small Amplitude Red Giants, or OSARGs. OSARGs have amplitudes of a few thousandths of a magnitude and semi-regular periods of 10 to 100 days. The OGLE survey published up to three periods for each OSARG, indicating a complex combination of pulsations. Many thousands of OSARGs were quickly detected in the Magellanic Clouds, both AGB and RGB stars. A catalog has since been published of 192,643 OSARGs in the direction of the Milky Way central bulge. Although around a quarter of Magellanic Cloud OSARgs show long secondary periods, very few of the galactic OSARGs do. The RGB OSARGs follow three