The Yellow Hypergiants HR 8752 and ρ Cassiopeiae near the Evolutionary Border of Instability

High-resolution near-ultraviolet spectra of the yellow hypergiants HR 8752 and ρ Cassiopeiae indicate high effective temperatures, placing both stars near the low-Teff border of the "yellow evolutionary void." At present, the temperature of HR 8752 is higher than ever. For this star we found Teff = 7900 ± 200 K, whereas ρ Cassiopeiae has Teff = 7300 ± 200 K. Both HR 8752 and ρ Cassiopeiae have developed strong stellar winds with V∞ ≃ 120 and V∞ ≃ 100 km s-1, respectively. For HR 8752 we estimate an upper limit for the spherically symmetric mass loss of 6.7 × 10-6 M☉ yr-1. Over the past decades two yellow hypergiants appear to have approached an evolutionary phase, which has never been observed before. We present the first spectroscopic evidence of the blueward motion of a cool super/hypergiant on the H-R diagram.