A pair of sub-Neptunes transiting the bright K-dwarf TOI-1064 characterized with <i>CHEOPS</i>

We report the discovery and characterisation of a pair sub-Neptunes transiting bright K-dwarf TOI-1064 (TIC 79748331), initially detected in TESS photometry. To characterise system, we performed retrieved CHEOPS, TESS, ground-based photometry, HARPS high-resolution spectroscopy, Gemini speckle imaging. host star determine $T_{\rm eff, \star}=4734\pm67$ K, $R_{\star}=0.726\pm0.007$ $R_{\odot}$, $M_{\star}=0.748\pm0.032$ $M_{\odot}$. present novel detrending method based on PSF shape-change modelling demonstrate its suitability to correct flux variations CHEOPS data. confirm planetary nature both bodies find that b has an orbital period $P_{\rm b}=6.44387\pm0.00003$ d, radius $R_{\rm b}=2.59\pm0.04$ $R_{\oplus}$, mass $M_{\rm b}=13.5_{-1.8}^{+1.7}$ $M_{\oplus}$, whilst c c}=12.22657^{+0.00005}_{-0.00004}$ c}=2.65\pm0.04$ 3$\sigma$ upper limit 8.5 ${\rm M_{\oplus}}$. From high-precision photometry obtain uncertainties $\sim$1.6%, allowing us conduct internal structure atmospheric escape modelling. is one densest, well-characterised sub-Neptunes, with tenuous atmosphere can be explained by loss primordial envelope following migration through protoplanetary disc. It likely extended due tentative low density, however further RVs are needed this scenario similar radii, different masses system. The data important for planets region mass-radius space, it allows identify trend bulk density-stellar metallicity massive may hint at formation population planets.