1901 - 2 .] Dr A. W. Boberts on Algol Variation. 
81 
Thus if 
p = ratio of diameter of S (1) to diameter of orbit 
q — ratio of diameter of S (2) to diameter of orbit 
m l = mass of S x 
m 2 = mass of S 2 
A x = density of S : 
A 2 = density of S 2 
t = time of revolution in solar days 
then 
and if p = q 
. *0135 
_-0135 
A t + A 2 = 
m 1 
m 1 + m 2 
m 2 
m 1 + m 2 
0135 
pH 2 . 
In the system we are considering, 
Therefore 
p = r — 0*163 
t = 1’854 days. 
Aj + A 2 _ 
- 2 = CP44 
that is, the mean density of the system is a little less than half 
that of the sun. 
It may be stated that this value is a good deal higher than is 
found for most of the other close binary systems. 
In the AstropJiysical Journal , vol. x. Ko. 5, I pointed out that 
the average density of Southern Algol binary stars is one-eighth 
that of the sun. A recent investigation, including a larger number 
of stars, yields a similar value. 
There is this important reservation, however, that if we consider 
only these stars that revolve round one another, practically in 
contact, we will obtain a much smaller mean density than if we 
take the mean density of all the Algol binaries. The importance 
of this fact in connection with the evolution of close binary systems 
will at once be evident. The mean density of binary stars that 
are in that early stage of their development when fission has 
taken or is taking place, will be more rare than the mean 
density of systems that have, through the reflex action of their 
PROC. ROY. SOC. EDIN. — VOL. XXIV. 6 
