Which sign has sometimes been used in sidereal astrology as a thirteenth Zodiac sign?

Ophiuchus has sometimes been used in sidereal astrology as a thirteenth sign in addition to the twelve signs of the tropical Zodiac, because the eponymous constellation Ophiuchus (Greek: Οφιούχος "Serpent-bearer"), as defined by the 1930 International Astronomical Union's constellation boundaries, is situated behind the sun from November 29 to December 18.

The idea appears to have originated in 1970 with Stephen Schmidt's suggestion of a 14-sign zodiac, also including Cetus as a sign. A 13-sign zodiac has been suggested by Walter Berg and by Mark Yazaki in 1995, a suggestion that achieved some popularity in Japan, where Ophiuchus is known as Hebitsukai-Za.

In sidereal and tropical astrology (including sun sign astrology), a 12-sign zodiac is used based on dividing the ecliptic into 12 equal parts rather than the IAU constellation boundaries. That is, astrological signs do not correspond to the constellations which are their namesakes, particularly not in the case of the tropical system where the divisions are fixed relative to the equinox, moving relative to the constellations.

There is much debate about Ophiuchus's status as a member of the zodiac. With details about it still unknown, such as its element and its placement among the core 12 zodiac members, it is usually considered to be a pseudo-member.