9 Cephei (9 Cep), also known as V337 Cephei, is a variable star in the constellation Cepheus.
9 Cephei was given the name V337 Cephei and classified as an α Cygni variable in 1967. It varies irregularly between magnitude 4.69 and 4.78. A study of the Hipparcos satellite photometry showed an amplitude of 0.56 magnitudes, but could find no periodicity.
9 Cephei is considered to be a member of the Cepheus OB2 stellar association, a scattering of massive bright stars around a thousand parsecs away in the southern part of the constellation Cepheus.
Calculations of the physical properties of 9 Cephei vary considerably even from broadly similar observational data. Modelling using the non-LTE line-blanketed CMFGEN atmospheric code gives a temperature of 18,000 K, radius of 40 R☉, luminosity of 151,000 L☉, and mass of 21 M☉. Calculations using the FASTWIND model give gives a temperature of 19,200 K, radius of 32 R☉, luminosity of 129,000 L☉, and mass of 12 M☉.