Extended CO line emission at 2.6 mm has been mapped in the Cepheus region over 490 sq deg at an angular resolution of 0.5 deg with the Columbia 1.2-m telescope. Two molecular clouds, representing a total of 130,000 solar masses have been found above the bulk of the Local arm at a distance of 800-900 pc. The observations also reveal a vast molecular complex at about 300 pc: one cloud in Cassiopeia stretches along a 30-deg arc and contains about 100,000 solar masses, and a second at a similar distance in Cepheus contains 60,000 solar masses. The arrangement of the nearby clouds in a ring 12-deg in diameter, the small amount of atomic and molecular emission inside the ring, the large H I and CO velocity dispersions along its rim, and soft X-ray and radio observations all strongly suggest the presence of a hot bubble between Cepheus and Cassiopeia. As a tentative interpretation, a 40,000-yr old Type I supernova remnant 300 pc away is proposed.
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Grenier et al. (1989) studied this question.