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## Period-Luminosity Relations of Variable Red Giant Stars

Period-luminosity (*PL*) relations of variable red giants in the Large
(LMC) and Small Magellanic Clouds (SMC) are presented. The *PL*
diagrams are plotted in three planes: log*P*-*K*_{S},
log*P*-*W*_{JK}, and log*P*-*W*_{I},
where *W*_{JK} and *W*_{I} are reddening free
Wesenheit indices. Fourteen *PL* sequences are distinguishable, and
some of them consist of three closely spaced ridges. Each of the sequences
is fitted with a linear or quadratic function. The similarities and
differences between the *PL* relations in both galaxies are discussed
for four types of red giant variability: OGLE Small Amplitude Red Giants
(OSARGs), Miras and Semiregular Variables (SRVs), Long Secondary Periods
(LSPs) and ellipsoidal variables.

We propose a new method of separating OSARGs from non-variable stars and
SRVs. The method employs the position in the reddening-free *PL*
diagrams and the characteristic period ratios of these multiperiodic
variables. The *PL* relations for the LMC OSARG are compared with the
calculated relations for RGB models along isochrones of relevant ages and
metallicities. We also compare measured periods and amplitudes of the
OSARGs with predictions based on the relations valid for less luminous
solar-like pulsators.

Miras and SRVs seem to follow *PL* relation of the same slopes in the
LMC and SMC, while for LSP and ellipsoidal variables slopes in both
galaxies are different. The *PL* sequences defined by LSP variables
and binary systems overlap in the whole range of analyzed wavebands. We put
forward new arguments for the binary star scenario as an explanation of the
LSP variability and elaborate on it further. The measured pulsation to
orbital period ratio implies nearly constant ratio of the star radius to
orbital distance, *R/A*=0.4, as we find. Combined effect of tidal
friction and mass loss enhanced by the low-mass companion may explain why
such a value is preferred.

The paper is available via
astro-ph/07102780

PLEASE cite the following paper when using the data or referring to these
OGLE results:

Soszynski *et al.*, 2007,
*Acta Astron.*, 57, 201.

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