SECULAR CHANGE IN THE PERIOD OF U CARINA. 



By Alexander William Roberts, D.Sc, F.R.A.S.. F.R.S.E. 



It has been known for well-nigh a century tliat the period 

 of some variable stars exhibits secular change of a very definite 

 character. Thus, in if^SS, Argelander found that the period 

 of /3 hyvx had l)een increasing during the years that it had 

 been under examination. 



The secular variation of a number of other stars has als«> 

 been determined by workers in this field of astronomical research. 



In the Southern Hemisphere continuous observations carried 

 on at Lovedale for twenty-five year have revealed this fact: that 

 the majority of short-period variable stars — that is. stars whose 

 period amounts to a few da\s, and whose variation is in some as 

 yet unknown way intiuu-itely related to l)inarv movement — is 

 subject to a secular change in period. 



In certain cases this secular change seems to be that of 

 regular acceleration or regular decrease ; change, no doubt, ex- 

 ceedingly minute, but yet a])preciable in the lapse of years. Thus, 

 if we imagine a star whose jX'riod is completed in five days 

 increasing only 



o.oooooi day, 

 every revolution, that is, one-tenth of a second, it would seem, at 

 first sight, that such a change would, with difficulty, be detected. 

 But when we deal with observations extending over a quarter 

 of a century, the accumulated acceleration becomes api)recia])le 

 and measurable. Thus, (;i5 periods during the first part of the 

 twent\'-four years would be completed in 4575.42 dav^, and dm-- 

 ing the second part in 4576.27 (la\'s. 



It is evident that minute changes in })eriod will thus be- 

 come readil)' measuraljle when the star lias been regular! \ 

 observed over an extended space of time. 



In the case of a few stars secular variation of a distincth 

 circular type has been observed. That is the period decreases, 

 increases, and then returns again to its starting point. 



The most remarkable star of this class is U Carin^e, whose 

 average period is 38.7647 days. Its range of variations is ()./ m. 

 to 7.8 m. It may be of interest briefly to state the nature and 

 extent of the variation to which this star's period is subjected. 



The variation of U Carin^e was discovered at Lovedale in 

 if^9i, and since then it has been under continuous observation up 

 to the present date. Its rise to maximum brightness is extremelv 

 rapid. Thus, instead of determining its period from maxima 

 points. I have preferred to determine it from dates when the 

 star passes through a definite brightness on its upward curve. 



The following table sets forth the data connected with the 

 maxima dates thus determined. Column i is the rotation num- 

 ber : column 2 gives the number of periods counting from the 



