Context : The soft X-ray emission from high density plasma observed in several CTTS is usually associated with the accretion process .
However , it is still unclear whether this high density “ cool ” plasma is heated in the accretion shock , or if it is coronal plasma fed or modified by the accretion process .

Aims : We conducted a coordinated quasi-simultaneous optical and X-ray observing campaign of the CTTS V2129 Oph .
In this paper we analyze Chandra grating spectrometer data and aim at correlating the observed X-ray emitting plasma components with the characteristics of the accretion process and of the stellar magnetic field constrained through simultaneous optical observations .

Methods : We analyze a 200 ks Chandra/HETGS observation , subdivided into two 100 ks segments , of the CTTS V2129 Oph .
For the two observing segments , corresponding to two different phases within one stellar rotation , we measure the density of the cool plasma component and the emission measure distribution .

Results : The X-ray emitting plasma covers a wide range of temperatures : from 2 up to 34 MK .
The cool plasma component of V2129 Oph ( T \approx 3 - 4 MK ) varies between the two segments of the Chandra observation : high density plasma ( \log N _ { e } = 12.1 ^ { +0.6 } _ { -1.1 } ) with high EM at \sim 3 - 4 MK is present during the first observing segment ; during the second segment this plasma component has lower EM and lower density ( \log N _ { e } < 11.5 ) , although the statistical significance of these differences is marginal .
Hotter plasma components , T \geq 10 MK , show variability on short time scales ( \sim 10 ks ) , typical of coronal plasma .
A clear flare , detected during the first segment , could be located in a large coronal loop ( half length > 3 R _ { \star } ) .

Conclusions : Our observation provides further confirmation that the dense cool plasma at a few MK in CTTS is material heated in the accretion shock .
The variability of this cool plasma component on V2129 Oph may be explained in terms of X-rays emitted in the accretion shock and seen with different viewing angles at the two rotational phases probed by our observation .
In particular , during the first time interval direct view of the shock region is possible , while , during the second , the accretion funnel itself intersects the line of sight to the shock region , preventing us from observing the accretion-driven X-rays .