SN 2000cx and SN 2013bh: extremely rare, nearly twin Type Ia supernovae

The Type Ia supernova (SN Ia) SN 2000cx was one of the most peculiar transients ever discovered, with a rise to maximum brightness typical of a SN Ia, but a slower decline and a higher photospheric temperature. 13 yr later SN 2013bh (also known as iPTF13abc), a near identical twin, was discovered and we obtained optical and near-infrared photometry and low-resolution optical spectroscopy from discovery until about 1 month past r-band maximum brightness. The spectra of both objects show iron-group elements [Co ii, Ni ii, Fe ii, Fe iii and high-velocity features (HVFs) of Ti ii], intermediate-mass elements (Si ii, Si iii and S ii) and separate normal velocity features (similar to 12 000 km s(-1)) and HVFs (similar to 24 000 km s(-1)) of Ca ii. Persistent absorption from Fe iii and Si iii, along with the colour evolution, implies high blackbody temperatures for SNe 2013bh and 2000cx (similar to 12 000 K). Both objects lack narrow Na i D absorption and exploded in the outskirts of their hosts, indicating that the SN environments were relatively free of interstellar or circumstellar material and may imply that the progenitors came from a relatively old and low-metallicity stellar population. Models of SN 2000cx, seemingly applicable to SN 2013bh, imply the production of up to similar to 1 M-circle dot of Ni-56 and (4.3-5.5) x 10(-3) M-circle dot of fast-moving Ca ejecta.