The lack of short baselines, referred to as the short-spacing problem (SSP), is a well-known limitation of the performance of radio interferometers, causing a reduction of the ux detected from source structure on large angular scales. The very large number of antennas operated in the Atacama Large Millimeter/sub-millimeter Array (ALMA) generates situations for which the impact of the SSP takes a complex form, not simply measurable by a single number, such as the maximal recoverable scale. In particular, extended antenna congurations, complemented by a small group of closeby antennas at the centre of the array, may result in a double-humped baseline distribution with a signicant gap between the two groups. In such cases one should adopt as the effective maximal recoverable scale the one associated with the extended array and use only the central array to recover missing ux, as one would do with single dish or ACA (Atacama Compact Array) observations. The impact of the missing baselines can be very important and may easily be underestimated, or even overlooked. The present study uses ALMA archival data of the 29SiO(8-7) line emission of the AGB star W Hydrae for a demonstration. A critical discussion of the reliability of the observations away from the star is presented together with comments of a broader scope. Properties of the circumstellar envelope of W Hya within 15 au from the star, many of which are not mentioned in the published literature, are brie y described and compared with R Doradus, an AGB star having properties very similar to W Hya.

Abstract
1. INTRODUCTION
2. OBSERVATIONS, DATA REDUCTION, AND IMAGING
3. IMPACT OF THE TWO-HUMPED BASELINEDISTRIBUTION
    3.1. General Considerations
    3.2. W Hya Observations: Radial Distributions
    3.3. W Hya Observations: Production of Artefacts
4. W HYA NEAR THE STELLAR DISC:A COMPARISON WITH R DOR
5. SUMMARY AND CONCLUSIONS
REFERENCES

• Do Thi Hoai(Department of Astrophysics, Vietnam National Space Center, Vietnam Academy of Science and Technology)
• Pham Tuan Anh(Department of Astrophysics, Vietnam National Space Center, Vietnam Academy of Science and Technology)
• Pham Tuyet Nhung(Department of Astrophysics, Vietnam National Space Center, Vietnam Academy of Science and Technology)
• Pierre Darriulat(Department of Astrophysics, Vietnam National Space Center, Vietnam Academy of Science and Technology)
• Pham Ngoc Diep(Department of Astrophysics, Vietnam National Space Center, Vietnam Academy of Science and Technology)
• Nguyen Bich Ngoc(Department of Astrophysics, Vietnam National Space Center, Vietnam Academy of Science and Technology/Graduate University of Science and Technology, Vietnam Academy of Science and Technology)
• Tran Thi Thai(Department of Astrophysics, Vietnam National Space Center, Vietnam Academy of Science and Technology/Graduate University of Science and Technology, Vietnam Academy of Science and Technology) Corresponding author