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		| Paper: | 
		On the Nature of the Clocks of Double Periodic Variables | 
	 
	
		| Volume: | 
		335, The Light-Time Effect in Astrophysics:  Causes and Cures of the O - C diagram | 
	 
	
		| Page: | 
		129 | 
	 
	
		| Authors: | 
		Mennickent, R.E.; Assmann, P.; Pietrzynski, G.; Gieren, W. | 
	 
	
	
		| Abstract: | 
		We analyze times of maximum and minimum in the long-term light curves of the recently discovered double periodic variables in order to enlighten the nature of their photometric variability. Whereas it seems well established that the short-term variability is strictly periodic and seems to be well explained as ellipsoidal variations in a close binary system, we find the long-term variability not strictly periodic, with an hitherto unexplained origin. | 
	 
	
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