We present the photometry of 16 91T/99aa-like Type Ia Supernovae (SNeIa) observed by the Las Cumbres Observatory. We also use an additional set of 21 91T/99aa-like SNe Ia and 87 normal SNe Ia from the literature for an analysis of the standardizability of the luminosity of 91T/99aa-like SNe. We find that 91T/99aa-like SNe are 0.2mag brighter than normal SNe Ia, even when fully corrected by the light-curve shapes and colors. The weighted rms of the 91T/99aa-like SNe (with z_CMB_>0.01) Hubble residuals is 0.25+/-0.03mag, suggesting that 91T/99aa-like SNe are also excellent relative distance indicators to +/-12%. We compare the Hubble residuals with the pseudo-equivalent width (pEW) of SiII{lambda}{lambda}6355 around the date of maximum brightness. We find that there is a broken linear correlation between those two measurements for our sample including both 91T/99aa-like and normal SNe Ia. As the pEW_max_(SiII{lambda}{lambda}6355) increases, the Hubble residual increases when pEW_max_(SiII{lambda}{lambda}6355)<55.6{AA}. However, the Hubble residual stays constant beyond this. Given that 91T/99aa-like SNe possess shallower SiII lines than normal SNe Ia, the linear correlation at pEW_max_(SiII{lambda}{lambda}6355)<55.6{AA} can account for the overall discrepancy of Hubble residuals derived from the two subgroups. Such a systematic effect needs to be taken into account when using SNe Ia to measure luminosity distances.

Cone search capability for table J/ApJ/938/83/table1 (General properties of Las Cumbres 91T/99aa-like supernovae)

Cone search capability for table J/ApJ/938/83/table2 (General properties of 91T/99aa-like SNe Ia in the literature)

Cone search capability for table J/ApJ/938/83/table8 (Parameters of normal SNe Ia in our sample)