Page "Extrasolar planet" Paragraph 49

If a planet orbits one member of a binary star system, then an uppercase letter for the star will be followed by a lowercase letter for the planet.

Examples are 16 Cygni Bb and HD 178911 Bb.

Planets orbiting the primary or " A " star should have ' Ab ' after the name of the system, as in HD 41004 Ab.

However, the " A " is sometimes omitted ; for example the first planet discovered around the primary star of the Tau Boötis binary system is usually called simply Tau Boötis b.