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UCAC2 46706450 is a late-type star with an ultraviolet (UV) excess. It wasconsidered a candidate to establish a sample of FGK stars with white dwarf (WD)companions that can be used to test binary evolution models. To verify the WDnature of the companion, UV spectroscopy was performed by Parsons et al.(2016). By a detailed model-atmosphere analysis we show that the UV source isan extremely hot WD with effective temperature $T_\mathrm{eff}$ =$105\,000\pm5000$ K, mass $M/M_\odot = 0.54\pm0.02$, radius $R/R_\odot =0.040^{+0.005}_{-0.004}$, and luminosity $L/L_\odot= 176^{+55}_{-49}$, i.e.,the compact object is just about to enter the WD cooling sequence.Investigating spectra of the cool star ($T_\mathrm{eff}$ = $4945\pm250$ K) wefound that it is a K-type subgiant with $M/M_\odot = 0.8-2.4$, $R/R_\odot =5.9^{+0.7}_{-0.5}$, and $L/L_\odot= 19^{+5}_{-5}$, that is rapidly rotatingwith $v \sin(i)=81$ km s$^{-1}$. Optical light curves reveal a period of twodays and an o-band peak-to-peak amplitude of 0.06 mag. We suggest, that it iscaused by stellar rotation in connection with star spots. With the radius weinfer an extremely high rotational velocity of$v_{\mathrm{rot}}=151^{+18}_{-13}$ km s$^{-1}$, thus marking the star as one ofthe most rapidly rotating subgiants known. This explains chromospheric activityobserved by H$\alpha$ emission and emission-line cores in CaII H and K as wellas NUV flux excess. From equal and constant radial velocities of the WD and theK subgiant as well as from a fit to the spectral energy distribution we inferthat they form a physical, wide though unresolved binary system. Bothcomponents exhibit similar metal abundances and show iron-group elements withslightly oversolar (up to 0.6 dex) abundance, meaning that atomic diffusion inthe WD atmosphere is not yet active due to a residual, weak radiation-drivenwind. (abridged)

An extremely hot white dwarf with a rapidly rotating K-type subgiant companion: UCAC2 46706450