V440 Per is a Population I Cepheid with the period of 7.57 day and low amplitude , almost sinusoidal light and radial velocity curves .
With no reliable data on the 1st harmonic , its pulsation mode identification remained controversial .
We obtained a radial velocity curve of V440 Per with our new high precision and high throughput Poznań Spectroscopic Telescope .
Our data reach the accuracy of 130 m/s per individual measurement and yield a secure detection of the 1st harmonic with the amplitude of A _ { 2 } = 140 \pm 15 m/s .
The velocity Fourier phase \phi _ { 21 } of V440 Per is inconsistent at the 7.25 \sigma level with those of the fundamental mode Cepheids , implying that the star must be an overtone Cepheid , as originally proposed by Kienzle et al .
( 13 ) .
Thus , V440 Per becomes the longest period Cepheid with the securely established overtone pulsations .
We show , that the convective nonlinear pulsation hydrocode can reproduce the Fourier parameters of V440 Per very well .
Requirement to match the observed properties of V440 Per constrains free parameters of the dynamical convection model used in the pulsation calculations , in particular the radiative losses parameter .