... emission

The reason Preibisch & Zinnecker (2002) do not trust H $_{\rm\alpha }$ as an indicator are its time variability and the fact that part of the H $_{\rm\alpha }$ flux can be of chromospheric origin. However variability should if anything tend to lower the significance of the difference between CTTS and WTTS and the chromospheric origin of part of the H $_{\rm\alpha }$ flux should at most produce an effect opposite to that observed: the sample of strong H $_{\rm\alpha }$ stars, interpreted as accreting stars, will be indeed contaminated by chromospherically active stars, which are also likely to have high coronal activity and thus $L_{\rm X}$.

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... spectrum

Even in the case of detectors with intrinsic spectral resolution, the low source counts practically prevent in many cases the determination from the data of a reliable spectral model.

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... classes

Differences in the intrinsic spectra would however be interesting "per se'' for the understanding of the physical mechanism that determines X-ray emission.

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... conversions

The effective temperature for spectral type A6 was modified from 8350 K to 8050. The former value seems to be at odds with the spectral type- $T_{\rm eff}$ relation; we suspect a typographical mistake.

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... band

Conversion factors to un-absorbed flux are computed using PIMMS (Portable, Interactive, Multi-Mission Simulator), version 3.0, available on-line at http://asc.harvard.edu/toolkit/ pimms.jsp

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...$EW_{\rm H\alpha} < 5$

We choose 5 instead of the more customary 10 as EW threshold because we so obtain a better distinction between activity indicators of the two classes. As noted in the introduction lowering the threshold could, if anything, result in a reduction of the inferred difference in activity levels; the most active WTTS might indeed have strong chromospheric H $_{\rm\alpha }$ emission and be mistaken for CTTS.

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