de-reddened to account for interstellar extinction, but the small change corresponds to records of the spectrum and are considered to be real. The colour then reddened dramatically to a B−V value of as much as 1.6 magnitudes in 1973, dropped rapidly to 0.02 by 2000, and has remained about constant since then. The detailed observations available since 1960 also show rapid colour variations of about 0.2 magnitudes on scales of 1–5 years super-imposed on the overall trends. The spectral type over the same period has changed from a G0 hypergiant at the start of the 20th century, to early K in 1973, then rapidly back to G0 by 1977, continuing to reach A6 Ia+ in 2011. These spectral types are compatible with the observed colour changes, indicating changes in the temperature of the star or its dense winds. The spectrum contains nitrogen and helium emission lines with unusual P Cygni profiles, including "inverse P Cygni" and double-peaked line profiles. Forbidden NII lines and a triple-peaked Hα line have strengthened dramatically since 1993, and the profiles have also changed indicating developments in circumstellar material probably ejected from the star. Properties It appears that HR 8752 is not just varying in brightness and fluctuating in temperature and size like most unstable stars, but is actually undergoing a secular evolutionary shift from cooler to hotter temperatures. The temperature can be estimated with some accuracy from the spectral and colour observations. The calculated effective temperature increased from 4,500K in 1900 to 5,000K in 1960. At that stage the luminosity was around and the radius . The star then varied erratically until 1973 when it rapidly expanded and cooled. A detailed spectral analysis in 1977 reported a temperature low of 4,000K, with a peak luminosity in 1976 of with a radius of . The surface gravity at this time was calculated to be log(g) = -2, indicating that the visible surface was effectively detached from the star. The star then rapidly returned to around its previous temperature of 5,000K, a luminosity of , and radius of . Starting in 1985, HR 8752 began a startling change, increasing in temperature to around 8,000K and decreasing in size to by 2000, with a luminosity of . Since then the physical parameters have been more stable although the stellar wind continues to change. The surface gravity has returned to a more normal value for a luminous supergiant near