Editing Semiregular variable star (section)

==Pulsation==
The semiregular variable stars, particularly the SRa and SRb sub-classes, are often grouped with the [[Mira variable]]s under the [[long-period variable]] heading.  In other situations, the term is expanded to cover almost all cool pulsating stars.  The semi-regular giant stars are closely related to the Mira variables: Mira stars generally pulsate in the [[fundamental mode]]; semiregular giants pulsate in one or more [[overtone]]s.<ref name=lsp/>

Photometric studies in the [[Large Magellanic Cloud]] looking for [[gravitational microlensing]] events have shown that essentially all cool evolved stars are variable, with the coolest stars showing very large amplitudes and warmer stars showing only micro-variations.  The semiregular variable stars fall on one of five main [[period-luminosity relationship]] sequences identified, differing from the Mira variables only in pulsating in an overtone mode.  The closely related OSARG ([[OGLE]] small amplitude red giant) variables pulsate in an unknown mode.<ref name=ogle/><ref name=pl/>

Many semiregular variables show long secondary periods around ten times the main pulsation period, with amplitudes of a few tenths of a magnitude at visual wavelengths.  The cause of the pulsations is not known.<ref name=lsp/>