1901-2.] Dr A. W. Roberts on Algol Variation.. 
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Certain considerations regarding Algol Variation, with 
special reference to C.P.D. — 41 0, 4511. By Alexander 
W. Roberts, D.Sc., E.R.A.S. (With Two Plates.) 
(Read November 4, 1901.) 
There has recently been discovered at the Royal Observatory. 
Cape Town, an interesting star of the Algol type of variation, 
thus making in all ten such stars at present known in the southern 
sky. 
The star is C.P.D. - 41° *45 1 1 . 
R.A. lOh. 16m. 44s. (1875). 
Dec. - 41° 43'-8. 
The importance attaching to variable stars of this well-defined 
type is due in some measure to the fact that we have certain 
knowledge of the causes which produce their light changes. We 
are able to go beyond the circumstances of their variation to the 
conditions, physical or otherwise, which have produced the 
phenomena observed. 
With many of the other types of stellar variation, our knowledge 
ends at our observations. 
Research, for example, has indicated no satisfactory explanation 
of long period variation, or of irregular variation ; while of theories 
to account for new stars there is no end. 
But the eclipse of one star by another affords a complete and at 
the same time an exceedingly simple explanation of Algol variation. 
If a close binary system revolves in an orbit coincident, or nearly 
so, with the line of sight, it is evident that each component will 
eclipse or occult the other every revolution. The character, 
extent, and duration of the eclipse will depend upon the form and 
position of the orbit in which the stars move, as well as on their 
relative size and brightness. It is this condition of orbital 
movement — proximity of the component . stars, and movement 
