Search for gravitational wave signals from known pulsars in LIGO-Virgo O3 data using the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mn>5</mml:mn><mml:mi>n</mml:mi></mml:mrow></mml:math>-vector ensemble method

The $5n$-vector ensemble method is a multiple test for the targeted search of continuous gravitational waves from an ensemble of known pulsars. This method can improve the detection probability by combining the results from individually undetectable pulsars if few signals are near the detection threshold. In this paper, we apply the $5n$-vector ensemble method to the O3 dataset from the LIGO and Virgo detectors considering an ensemble of 201 known pulsars. We find no evidence for a signal from the ensemble and set a 95% credible upper limit on the mean ellipticity assuming a common exponential distribution for the pulsars' ellipticities. Using two independent hierarchical Bayesian procedures, we find upper limits of $1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ and $2.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ on the mean ellipticity for the 201 analyzed pulsars.