We present the first extensive photometric results of CL Aur from our BVRI CCD photometry made on 22 nights from 2003 November through 2005 February .
Fifteen new timings of minimum light were obtained .
During the past 104 years , the orbital period has varied due to a periodic oscillation superposed on a continuous period increase .
The period and semi-amplitude of the oscillation are about 21.6 yrs and 0.0133 d , respectively .
This detail is interpreted as a light-travel-time effect due to a low-luminosity K-type star gravitationally bound to the CL Aur close system .
Our photometric study indicates that CL Aur is a relatively short-period Algol-type binary with values of q =0.602 and i =88 ^ { \circ } .2 .
Mass transfer from the secondary to the primary eclipsing component is at least partly responsible for the observed secular period change with a rate of dP / dt = +1.4 \times 10 ^ { -7 } d yr ^ { -1 } .
A cool spot model has been calculated but we think that an alternative hot-spot model resulting from a gas stream impact on the hot star is more reasonable despite two difficulties with the explanation .
Absolute dimensions of the eclipsing system are deduced and its present state is compared with tracks for single star and conservative close binary evolution .
Finally , we examine the possible reconciliation of two different calculations of the luminosity of the hot spot and a re-interpretation of the secular term of the period variability .