•2.— NOTE ON THE INCREASING PERIOD OF CERTAIN 

 CLOSE BINARY STARS. 



By Alexaxdkk W. Robkkts, D.Sc, F.R.A.S., F.H.S.E. 



In a paper publisliefl in The Observatori/, No. .■>67, p. 98, I called 

 attention to the variation in the light period of f^ Lyrae, and suggested 

 that the steady increase in the period of this notable star was due in 

 whole or in part to tidal friction. 



As is well known, the close proximity of one body to another, i.e. 

 the earth and the moon, induces tides, and one effect of these tides 

 is to diminish the rotational rate of the system, and consequently 

 to increase the distance between the component members of the 

 system. 



The )-elation existing between the period and the distance of any 

 two bodies of a system enables one readily to compute the rate of 

 recession when the amount of increase in period is known. It was 

 after this manner that I dealt with the variation of /3 Lyrae in the 

 paper referred to. 



It ma)'^ not be out of place to indicate here what were the main 

 contentions urged. As far back as 18.58 Argelander proved beyond a 

 doubt that the period of the variable star /3 Lyrae Avas steadily in- 

 creasing. His deductions gave the following values : — 



Date. 



d. 



1801-5 ... ... ... 12 



l«22-7 „ 



1827-2 „ 



1838-6 ... ... ... „ 



1850 „ „ 



On these figures Argelander remarks : " His numeris non solum 

 periodum revera cum tempore variabilem esse ultra omne dubium 

 ponitur, sed etiam quantitatem huius variationis ex observationibus 

 meis solis inventam proxime ad veritatem accedere arguitur." Since 

 Argelander's day many astronomers have redetermined the period of 

 /5 Lyrae as^^well as its secular variation. 



A suspicion that a few southern stars were aiFected with a similar 

 secular lengthening of period to /3 Lyrae led me in 1905 to make a 

 careful discussion of all such data as would lead to a definite deter- 

 mination of the amount by which the period of B Lyrae was annually 

 augmented. 



