Abstract
Observations of a new time of maximum light of CY Aqr, an extensively-observed SX Phoenicis star, have been investigated in combination with the data presented in the literature for almost 7 decades. The O - C diagram reveals that the long-term period change of CY Aqr can be represented by a linear period change combined with the light-time effect in a highly-eccentric orbit with a period of half a century. The binary nature of CY Aqr is therefore discussed with emphasis on the light-time effect procedure.