What does CY or VY signify?

Answer 1

It means they are variable stars and are named using a variation on the Bayer designation format of an identifying label combined with theLatin genitive of the name of the constellation in which the star lies.

So:

VY Canis Majoris = VY (Variable) Canis Majoris (Constellation)

YZ Ceti = YZ (Variable) Cetus (Constellation)

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Answer 2

CY typically stands for "calendar year" and VY typically stands for "fiscal year". Both terms are used to indicate the time period over which financial or operational data is being reported.