Context : When crossing the Hertzsprung gap , intermediate-mass stars develop a convective envelope .
Fast rotators on the main sequence , or Ap star descendants , are expected to become magnetic active subgiants during this evolutionary phase .

Aims : We compare the surface magnetic fields and activity indicators of two active , fast rotating red giants with similar masses and spectral class but different rotation rates - OU And ( P _ { rot } =24.2 d ) and 31 Com ( P _ { rot } =6.8 d ) - to address the question of the origin of their magnetism and high activity .

Methods : Observations were carried out with the Narval spectropolarimeter in 2008 and 2013 .
We used the least squares deconvolution ( LSD ) technique to extract Stokes V and I profiles with high S/N to detect Zeeman signatures of the magnetic field of the stars .
We then provide Zeeman-Doppler imaging ( ZDI ) , activity indicators monitoring , and a precise estimation of stellar parameters .
We use state-of-the-art stellar evolutionary models , including rotation , to infer the evolutionary status of our giants , as well as their initial rotation velocity on the main sequence , and we interpret our observational results in the light of the theoretical Rossby numbers .

Results : The detected magnetic field of OU Andromedae ( OU And ) is a strong one .
Its longitudinal component B _ { l } reaches 40 G and presents an about sinusoidal variation with reversal of the polarity .
The magnetic topology of OU And is dominated by large-scale elements and is mainly poloidal with an important dipole component , as well as a significant toroidal component .
The detected magnetic field of 31 Comae ( 31 Com ) is weaker , with a magnetic map showing a more complex field geometry , and poloidal and toroidal components of equal contributions .
The evolutionary models show that the progenitors of OU And and 31 Com must have been rotating at velocities that correspond to 30 and 53 \% , respectively , of their critical rotation velocity on the zero age main sequence .
Both OU And and 31 Com have very similar masses ( 2.7 and 2.85 M _ { \odot } , respectively ) , and they both lie in the Hertzsprung gap .

Conclusions : OU And appears to be the probable descendant of a magnetic Ap star , and 31 Com the descendant of a relatively fast rotator on the main sequence .
Because of the relatively fast rotation in the Hertzsprung gap and the onset of the development of a convective envelope , OU And also has a dynamo in operation .